diff --git a/libcxx/docs/Status/RangesAlgorithms.csv b/libcxx/docs/Status/RangesAlgorithms.csv
--- a/libcxx/docs/Status/RangesAlgorithms.csv
+++ b/libcxx/docs/Status/RangesAlgorithms.csv
@@ -51,8 +51,8 @@
 Write,remove_copy_if,Nikolas Klauser,n/a,Not started
 Write,replace,Nikolas Klauser,`D126283 <https://llvm.org/D126283>`_,✅
 Write,replace_if,Nikolas Klauser,`D126283 <https://llvm.org/D126283>`_,✅
-Write,replace_copy,Nikolas Klauser,n/a,Not started
-Write,replace_copy_if,Nikolas Klauser,n/a,Not started
+Write,replace_copy,Nikolas Klauser,`D126283 <https://llvm.org/D129806>`_,✅
+Write,replace_copy_if,Nikolas Klauser,`D126283 <https://llvm.org/D129806>`_,✅
 Write,swap_ranges,Nikolas Klauser,`D116303 <https://llvm.org/D116303>`_,✅
 Write,reverse_copy,Nikolas Klauser,`D127211 <https://llvm.org/D127211>`_,✅
 Write,rotate_copy,Nikolas Klauser,`D127211 <https://llvm.org/D127211>`_,✅
diff --git a/libcxx/include/__algorithm/ranges_replace_copy.h b/libcxx/include/__algorithm/ranges_replace_copy.h
--- a/libcxx/include/__algorithm/ranges_replace_copy.h
+++ b/libcxx/include/__algorithm/ranges_replace_copy.h
@@ -10,19 +10,16 @@
 #define _LIBCPP___ALGORITHM_RANGES_REPLACE_COPY_H
 
 #include <__algorithm/in_out_result.h>
-#include <__algorithm/make_projected.h>
-#include <__algorithm/replace_copy.h>
+#include <__algorithm/ranges_replace_copy_if.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__functional/invoke.h>
 #include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
-#include <__iterator/iterator_traits.h>
 #include <__iterator/projected.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/dangling.h>
-#include <__utility/forward.h>
 #include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -40,35 +37,45 @@
 
 namespace __replace_copy {
 
-struct __fn {
-
-  template <input_iterator _InIter, sentinel_for<_InIter> _Sent, class _Type1, class _Type2,
-            output_iterator<const _Type2&> _OutIter, class _Proj = identity>
-  requires indirectly_copyable<_InIter, _OutIter> &&
-           indirect_binary_predicate<ranges::equal_to, projected<_InIter, _Proj>, const _Type1*>
-  _LIBCPP_HIDE_FROM_ABI constexpr
-  replace_copy_result<_InIter, _OutIter>
-  operator()(_InIter __first, _Sent __last, _OutIter __result, const _Type1& __old_value, const _Type2& __new_value,
+  struct __fn {
+    template <input_iterator _InIter,
+              sentinel_for<_InIter> _Sent,
+              class _OldType,
+              class _NewType,
+              output_iterator<const _NewType&> _OutIter,
+              class _Proj = identity>
+      requires indirectly_copyable<_InIter, _OutIter> &&
+               indirect_binary_predicate<ranges::equal_to, projected<_InIter, _Proj>, const _OldType*>
+    _LIBCPP_HIDE_FROM_ABI constexpr replace_copy_result<_InIter, _OutIter>
+    operator()(_InIter __first,
+               _Sent __last,
+               _OutIter __result,
+               const _OldType& __old_value,
+               const _NewType& __new_value,
                _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__first; (void)__last; (void)__result; (void)__old_value; (void)__new_value; (void)__proj;
-    return {};
-  }
-
-  template <input_range _Range, class _Type1, class _Type2, output_iterator<const _Type2&> _OutIter,
-            class _Proj = identity>
-  requires indirectly_copyable<iterator_t<_Range>, _OutIter> &&
-           indirect_binary_predicate<ranges::equal_to, projected<iterator_t<_Range>, _Proj>, const _Type1*>
-  _LIBCPP_HIDE_FROM_ABI constexpr
-  replace_copy_result<borrowed_iterator_t<_Range>, _OutIter>
-  operator()(_Range&& __range, _OutIter __result, const _Type1& __old_value, const _Type2& __new_value,
+      auto __pred = [&](const auto& __value) { return __value == __old_value; };
+      return ranges::__replace_copy_if_impl(
+          std::move(__first), std::move(__last), std::move(__result), __pred, __new_value, __proj);
+    }
+
+    template <input_range _Range,
+              class _OldType,
+              class _NewType,
+              output_iterator<const _NewType&> _OutIter,
+              class _Proj = identity>
+      requires indirectly_copyable<iterator_t<_Range>, _OutIter> &&
+               indirect_binary_predicate<ranges::equal_to, projected<iterator_t<_Range>, _Proj>, const _OldType*>
+    _LIBCPP_HIDE_FROM_ABI constexpr replace_copy_result<borrowed_iterator_t<_Range>, _OutIter>
+    operator()(_Range&& __range,
+               _OutIter __result,
+               const _OldType& __old_value,
+               const _NewType& __new_value,
                _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__range; (void)__result; (void)__old_value; (void)__new_value; (void)__proj;
-    return {};
-  }
-
-};
+      auto __pred = [&](const auto& __value) { return __value == __old_value; };
+      return ranges::__replace_copy_if_impl(
+          ranges::begin(__range), ranges::end(__range), std::move(__result), __pred, __new_value, __proj);
+    }
+  };
 
 } // namespace __replace_copy
 
diff --git a/libcxx/include/__algorithm/ranges_replace_copy_if.h b/libcxx/include/__algorithm/ranges_replace_copy_if.h
--- a/libcxx/include/__algorithm/ranges_replace_copy_if.h
+++ b/libcxx/include/__algorithm/ranges_replace_copy_if.h
@@ -10,19 +10,14 @@
 #define _LIBCPP___ALGORITHM_RANGES_REPLACE_COPY_IF_H
 
 #include <__algorithm/in_out_result.h>
-#include <__algorithm/make_projected.h>
-#include <__algorithm/replace_copy_if.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__functional/invoke.h>
-#include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
-#include <__iterator/iterator_traits.h>
 #include <__iterator/projected.h>
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/dangling.h>
-#include <__utility/forward.h>
 #include <__utility/move.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -38,34 +33,49 @@
 template <class _InIter, class _OutIter>
 using replace_copy_if_result = in_out_result<_InIter, _OutIter>;
 
-namespace __replace_copy_if {
-
-struct __fn {
-
-  template <input_iterator _InIter, sentinel_for<_InIter> _Sent, class _Type, output_iterator<const _Type&> _OutIter,
-            class _Proj = identity, indirect_unary_predicate<projected<_InIter, _Proj>> _Pred>
-  requires indirectly_copyable<_InIter, _OutIter>
-  _LIBCPP_HIDE_FROM_ABI constexpr
-  replace_copy_if_result<_InIter, _OutIter>
-  operator()(_InIter __first, _Sent __last, _OutIter __result, _Pred __pred, const _Type& __new_value,
-             _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__first; (void)__last; (void)__result; (void)__pred; (void)__new_value; (void)__proj;
-    return {};
+template <class _InIter, class _Sent, class _OutIter, class _Pred, class _Type, class _Proj>
+_LIBCPP_HIDE_FROM_ABI constexpr replace_copy_if_result<_InIter, _OutIter> __replace_copy_if_impl(
+    _InIter __first, _Sent __last, _OutIter __result, _Pred& __pred, const _Type& __new_value, _Proj& __proj) {
+  while (__first != __last) {
+    if (std::invoke(__pred, std::invoke(__proj, *__first)))
+      *__result = __new_value;
+    else
+      *__result = *__first;
+    ++__first;
+    ++__result;
   }
+  return {std::move(__first), std::move(__result)};
+}
 
-  template <input_range _Range, class _Type, output_iterator<const _Type&> _OutIter, class _Proj = identity,
-            indirect_unary_predicate<projected<iterator_t<_Range>, _Proj>> _Pred>
-  requires indirectly_copyable<iterator_t<_Range>, _OutIter>
-  _LIBCPP_HIDE_FROM_ABI constexpr
-  replace_copy_if_result<borrowed_iterator_t<_Range>, _OutIter>
-  operator()(_Range&& __range, _OutIter __result, _Pred __pred, const _Type& __new_value, _Proj __proj = {}) const {
-    // TODO: implement
-    (void)__range; (void)__result; (void)__pred; (void)__new_value; (void)__proj;
-    return {};
-  }
+namespace __replace_copy_if {
 
-};
+  struct __fn {
+    template <input_iterator _InIter,
+              sentinel_for<_InIter> _Sent,
+              class _Type,
+              output_iterator<const _Type&> _OutIter,
+              class _Proj = identity,
+              indirect_unary_predicate<projected<_InIter, _Proj>> _Pred>
+      requires indirectly_copyable<_InIter, _OutIter>
+    _LIBCPP_HIDE_FROM_ABI constexpr replace_copy_if_result<_InIter, _OutIter> operator()(
+        _InIter __first, _Sent __last, _OutIter __result, _Pred __pred, const _Type& __new_value, _Proj __proj = {})
+        const {
+      return ranges::__replace_copy_if_impl(
+          std::move(__first), std::move(__last), std::move(__result), __pred, __new_value, __proj);
+    }
+
+    template <input_range _Range,
+              class _Type,
+              output_iterator<const _Type&> _OutIter,
+              class _Proj = identity,
+              indirect_unary_predicate<projected<iterator_t<_Range>, _Proj>> _Pred>
+      requires indirectly_copyable<iterator_t<_Range>, _OutIter>
+    _LIBCPP_HIDE_FROM_ABI constexpr replace_copy_if_result<borrowed_iterator_t<_Range>, _OutIter>
+    operator()(_Range&& __range, _OutIter __result, _Pred __pred, const _Type& __new_value, _Proj __proj = {}) const {
+      return ranges::__replace_copy_if_impl(
+          ranges::begin(__range), ranges::end(__range), std::move(__result), __pred, __new_value, __proj);
+    }
+  };
 
 } // namespace __replace_copy_if
 
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -703,7 +703,7 @@
       set_symmetric_difference(I1 first1, S1 last1, I2 first2, S2 last2, O result,
                                Comp comp = {}, Proj1 proj1 = {},
                                Proj2 proj2 = {});                                                   // since C++20
-  
+
   template<input_range R1, input_range R2, weakly_incrementable O,
            class Comp = ranges::less, class Proj1 = identity, class Proj2 = identity>
     requires mergeable<iterator_t<R1>, iterator_t<R2>, O, Comp, Proj1, Proj2>
@@ -711,7 +711,7 @@
                                                       borrowed_iterator_t<R2>, O>
       set_symmetric_difference(R1&& r1, R2&& r2, O result, Comp comp = {},
                                Proj1 proj1 = {}, Proj2 proj2 = {});                                 // since C++20
-  
+
   template<class I1, class I2, class O>
     using set_union_result = in_in_out_result<I1, I2, O>;                                           // since C++20
 
@@ -729,6 +729,38 @@
     constexpr set_union_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>, O>
       set_union(R1&& r1, R2&& r2, O result, Comp comp = {},
                 Proj1 proj1 = {}, Proj2 proj2 = {});                                                // since C++20
+
+  template<input_iterator I, sentinel_for<I> S, class T1, class T2,
+           output_iterator<const T2&> O, class Proj = identity>
+    requires indirectly_copyable<I, O> &&
+             indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*>
+    constexpr replace_copy_result<I, O>
+      replace_copy(I first, S last, O result, const T1& old_value, const T2& new_value,
+                   Proj proj = {});                                                                // Since C++20
+
+  template<input_range R, class T1, class T2, output_iterator<const T2&> O,
+           class Proj = identity>
+    requires indirectly_copyable<iterator_t<R>, O> &&
+             indirect_binary_predicate<ranges::equal_to,
+                                       projected<iterator_t<R>, Proj>, const T1*>
+    constexpr replace_copy_result<borrowed_iterator_t<R>, O>
+      replace_copy(R&& r, O result, const T1& old_value, const T2& new_value,
+                   Proj proj = {});                                                                // Since C++20
+
+  template<input_iterator I, sentinel_for<I> S, class T, output_iterator<const T&> O,
+           class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred>
+    requires indirectly_copyable<I, O>
+    constexpr replace_copy_if_result<I, O>
+      replace_copy_if(I first, S last, O result, Pred pred, const T& new_value,
+                      Proj proj = {});                                                             // Since C++20
+
+  template<input_range R, class T, output_iterator<const T&> O, class Proj = identity,
+           indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
+    requires indirectly_copyable<iterator_t<R>, O>
+    constexpr replace_copy_if_result<borrowed_iterator_t<R>, O>
+      replace_copy_if(R&& r, O result, Pred pred, const T& new_value,
+                      Proj proj = {});                                                             // Since C++20
+
 }
 
     constexpr bool     // constexpr in C++20
@@ -1501,6 +1533,8 @@
 #include <__algorithm/ranges_remove.h>
 #include <__algorithm/ranges_remove_if.h>
 #include <__algorithm/ranges_replace.h>
+#include <__algorithm/ranges_replace_copy.h>
+#include <__algorithm/ranges_replace_copy_if.h>
 #include <__algorithm/ranges_replace_if.h>
 #include <__algorithm/ranges_reverse.h>
 #include <__algorithm/ranges_reverse_copy.h>
diff --git a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
@@ -191,8 +191,8 @@
     //(void)std::ranges::remove_copy_if(a, first2, UnaryTrue(&copies)); assert(copies == 0);
     (void)std::ranges::remove_if(first, last, UnaryTrue(&copies)); assert(copies == 0);
     (void)std::ranges::remove_if(a, UnaryTrue(&copies)); assert(copies == 0);
-    //(void)std::ranges::replace_copy_if(first, last, first2, UnaryTrue(&copies), value); assert(copies == 0);
-    //(void)std::ranges::replace_copy_if(a, first2, UnaryTrue(&copies), value); assert(copies == 0);
+    (void)std::ranges::replace_copy_if(first, last, first2, UnaryTrue(&copies), value); assert(copies == 0);
+    (void)std::ranges::replace_copy_if(a, first2, UnaryTrue(&copies), value); assert(copies == 0);
     (void)std::ranges::replace_if(first, last, UnaryTrue(&copies), value); assert(copies == 0);
     (void)std::ranges::replace_if(a, UnaryTrue(&copies), value); assert(copies == 0);
     (void)std::ranges::search(first, last, first2, mid2, Equal(&copies)); assert(copies == 0);
diff --git a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
--- a/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
@@ -178,10 +178,10 @@
     (void)std::ranges::remove(a, value, Proj(&copies)); assert(copies == 0);
     (void)std::ranges::remove_if(first, last, UnaryTrue(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::remove_if(a, UnaryTrue(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::replace_copy(first, last, first2, value, T(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::replace_copy(a, first2, value, T(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::replace_copy_if(first, last, first2, UnaryTrue(), T(), Proj(&copies)); assert(copies == 0);
-    //(void)std::ranges::replace_copy_if(a, first2, UnaryTrue(), T(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::replace_copy(first, last, first2, value, T(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::replace_copy(a, first2, value, T(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::replace_copy_if(first, last, first2, UnaryTrue(), T(), Proj(&copies)); assert(copies == 0);
+    (void)std::ranges::replace_copy_if(a, first2, UnaryTrue(), T(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::replace(first, last, value, T(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::replace(a, value, T(), Proj(&copies)); assert(copies == 0);
     (void)std::ranges::replace_if(first, last, UnaryTrue(), T(), Proj(&copies)); assert(copies == 0);
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy.pass.cpp
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy.pass.cpp
@@ -26,8 +26,6 @@
 //     replace_copy(R&& r, O result, const T1& old_value, const T2& new_value,
 //                  Proj proj = {});                                                                // Since C++20
 
-// TODO: synopsis
-
 #include <algorithm>
 #include <array>
 #include <concepts>
@@ -37,12 +35,185 @@
 #include "almost_satisfies_types.h"
 #include "test_iterators.h"
 
-// TODO: SFINAE tests.
+template <class Iter, class Sent = sentinel_wrapper<Iter>, class OutIter = int*>
+concept HasRemoveIfIt =
+    requires(Iter first, Sent last, OutIter result) { std::ranges::replace_copy(first, last, result, 0, 0); };
+
+static_assert(HasRemoveIfIt<int*>);
+static_assert(!HasRemoveIfIt<InputIteratorNotDerivedFrom>);
+static_assert(!HasRemoveIfIt<InputIteratorNotIndirectlyReadable>);
+static_assert(!HasRemoveIfIt<InputIteratorNotInputOrOutputIterator>);
+static_assert(!HasRemoveIfIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasRemoveIfIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(!HasRemoveIfIt<int*, int*, OutputIteratorNotIndirectlyWritable>);
+static_assert(!HasRemoveIfIt<int*, int*, OutputIteratorNotInputOrOutputIterator>);
+
+template <class Range, class OutIter = int*>
+concept HasReplaceCopyR = requires(Range range, OutIter result) { std::ranges::replace_copy(range, result, 0, 0); };
+
+static_assert(HasReplaceCopyR<UncheckedRange<int*>>);
+static_assert(!HasReplaceCopyR<InputRangeNotDerivedFrom>);
+static_assert(!HasReplaceCopyR<InputRangeNotIndirectlyReadable>);
+static_assert(!HasReplaceCopyR<InputRangeNotInputOrOutputIterator>);
+static_assert(!HasReplaceCopyR<InputRangeNotSentinelSemiregular>);
+static_assert(!HasReplaceCopyR<InputRangeNotSentinelEqualityComparableWith>);
+static_assert(!HasReplaceCopyR<UncheckedRange<int*>, OutputIteratorNotIndirectlyWritable>);
+static_assert(!HasReplaceCopyR<UncheckedRange<int*>, OutputIteratorNotInputOrOutputIterator>);
+
+template <int N>
+struct Data {
+  std::array<int, N> input;
+  int old_value;
+  int new_value;
+  std::array<int, N> expected;
+};
+
+template <class InIter, class Sent, class OutIter, int N>
+constexpr void test(Data<N> d) {
+  { // iterator overload
+    std::array<int, N> output;
+
+    auto first  = InIter(d.input.data());
+    auto last   = Sent(InIter(d.input.data() + d.input.size()));
+    auto result = OutIter(output.data());
+
+    std::same_as<std::ranges::in_out_result<InIter, OutIter>> decltype(auto) ret =
+        std::ranges::replace_copy(std::move(first), std::move(last), std::move(result), d.old_value, d.new_value);
+    assert(base(ret.in) == d.input.data() + d.input.size());
+    assert(base(ret.out) == output.data() + output.size());
+    assert(d.expected == output);
+  }
+  { // range overload
+    std::array<int, N> output;
+
+    auto range  = std::ranges::subrange(InIter(d.input.data()), Sent(InIter(d.input.data() + d.input.size())));
+    auto result = OutIter(output.data());
+
+    std::same_as<std::ranges::in_out_result<InIter, OutIter>> decltype(auto) ret =
+        std::ranges::replace_copy(range, result, d.old_value, d.new_value);
+    assert(base(ret.in) == d.input.data() + d.input.size());
+    assert(base(ret.out) == output.data() + output.size());
+    assert(d.expected == output);
+  }
+}
+
+template <class InIter, class Sent, class OutIter>
+constexpr void tests() {
+  // simple test
+  test<InIter, Sent, OutIter, 4>({.input = {1, 2, 3, 4}, .old_value = 2, .new_value = 5, .expected = {1, 5, 3, 4}});
+  // empty range
+  test<InIter, Sent, OutIter, 0>({.input = {}, .old_value = 2, .new_value = 5, .expected = {}});
+  // all elements match
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .old_value = 1, .new_value = 2, .expected = {2, 2, 2, 2}});
+  // no element matches
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .old_value = 2, .new_value = 3, .expected = {1, 1, 1, 1}});
+  // old_value and new_value are identical - match
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .old_value = 1, .new_value = 1, .expected = {1, 1, 1, 1}});
+  // old_value and new_value are identical - no match
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .old_value = 2, .new_value = 2, .expected = {1, 1, 1, 1}});
+  // more elements
+  test<InIter, Sent, OutIter, 7>(
+      {.input = {1, 2, 3, 4, 5, 6, 7}, .old_value = 2, .new_value = 3, .expected = {1, 3, 3, 4, 5, 6, 7}});
+  // single element - match
+  test<InIter, Sent, OutIter, 1>({.input = {1}, .old_value = 1, .new_value = 5, .expected = {5}});
+  // single element - no match
+  test<InIter, Sent, OutIter, 1>({.input = {2}, .old_value = 1, .new_value = 5, .expected = {2}});
+}
+
+template <class InIter, class Sent>
+constexpr void test_output_iterators() {
+  tests<InIter, Sent, cpp17_output_iterator<int*>>();
+  tests<InIter, Sent, forward_iterator<int*>>();
+  tests<InIter, Sent, bidirectional_iterator<int*>>();
+  tests<InIter, Sent, random_access_iterator<int*>>();
+  tests<InIter, Sent, contiguous_iterator<int*>>();
+  tests<InIter, Sent, int*>();
+}
+
+template <class InIter>
+constexpr void test_sentinels() {
+  test_output_iterators<InIter, InIter>();
+  test_output_iterators<InIter, sentinel_wrapper<InIter>>();
+  test_output_iterators<InIter, sized_sentinel<InIter>>();
+}
 
 constexpr bool test() {
-  // TODO: main tests.
-  // TODO: A custom comparator works.
-  // TODO: A custom projection works.
+  test_output_iterators<cpp17_input_iterator<int*>, sentinel_wrapper<cpp17_input_iterator<int*>>>();
+  test_output_iterators<cpp20_input_iterator<int*>, sentinel_wrapper<cpp20_input_iterator<int*>>>();
+  test_sentinels<forward_iterator<int*>>();
+  test_sentinels<bidirectional_iterator<int*>>();
+  test_sentinels<random_access_iterator<int*>>();
+  test_sentinels<contiguous_iterator<int*>>();
+  test_sentinels<int*>();
+  test_sentinels<const int*>();
+
+  { // check that a custom projection works
+    struct S {
+      int i;
+    };
+    { // iterator overload
+      S a[] = {{1}, {2}, {3}, {4}};
+      S b[4];
+      auto ret = std::ranges::replace_copy(std::begin(a), std::end(a), std::begin(b), 1, S{2}, &S::i);
+      assert(ret.in == std::end(a));
+      assert(ret.out == std::end(b));
+    }
+    { // range overload
+      S a[] = {{1}, {2}, {3}, {4}};
+      S b[4];
+      auto ret = std::ranges::replace_copy(a, std::begin(b), 1, S{2}, &S::i);
+      assert(ret.in == std::end(a));
+      assert(ret.out == std::end(b));
+    }
+  }
+
+  {   // check complexity requirements
+    { // iterator overload
+      int proj_count = 0;
+      auto proj      = [&](int i) {
+        ++proj_count;
+        return i;
+      };
+      int a[] = {1, 2, 3, 4};
+      int b[4];
+      std::ranges::replace_copy(std::begin(a), std::end(a), std::begin(b), 0, 0, proj);
+      assert(proj_count == 4);
+    }
+    { // range overload
+      int proj_count = 0;
+      auto proj      = [&](int i) {
+        ++proj_count;
+        return i;
+      };
+      int a[] = {1, 2, 3, 4};
+      int b[4];
+      std::ranges::replace_copy(a, std::begin(b), 0, 0, proj);
+      assert(proj_count == 4);
+    }
+  }
+
+  { // using different types the old and new value works
+    struct S {
+      constexpr operator int() const { return 0; }
+      constexpr bool operator==(const S&) const = default;
+      constexpr bool operator==(int i) const { return i == 0; }
+    };
+    struct T {
+      constexpr operator int() const { return 1; }
+    };
+    {
+      int a[] = {0, 1, 2, 3};
+      int b[4];
+      std::ranges::replace_copy(std::begin(a), std::end(a), std::begin(b), S{}, T{});
+      assert(std::ranges::equal(b, std::array{1, 1, 2, 3}));
+    }
+    {
+      int a[] = {0, 1, 2, 3};
+      int b[4];
+      std::ranges::replace_copy(a, std::begin(b), S{}, T{});
+      assert(std::ranges::equal(b, std::array{1, 1, 2, 3}));
+    }
+  }
 
   return true;
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy_if.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy_if.pass.cpp
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy_if.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.replace/ranges_replace_copy_if.pass.cpp
@@ -34,12 +34,187 @@
 #include "almost_satisfies_types.h"
 #include "test_iterators.h"
 
-// TODO: SFINAE tests.
+struct FalsePredicate {
+  constexpr bool operator()(int) { return false; }
+};
+
+template <class Iter, class Sent = sentinel_wrapper<Iter>, class OutIter = int*>
+concept HasRemoveIfIt = requires(
+    Iter first, Sent last, OutIter result) { std::ranges::replace_copy_if(first, last, result, FalsePredicate{}, 0); };
+
+static_assert(HasRemoveIfIt<int*>);
+static_assert(!HasRemoveIfIt<InputIteratorNotDerivedFrom>);
+static_assert(!HasRemoveIfIt<InputIteratorNotIndirectlyReadable>);
+static_assert(!HasRemoveIfIt<InputIteratorNotInputOrOutputIterator>);
+static_assert(!HasRemoveIfIt<int*, SentinelForNotSemiregular>);
+static_assert(!HasRemoveIfIt<int*, SentinelForNotWeaklyEqualityComparableWith>);
+static_assert(!HasRemoveIfIt<int*, int*, OutputIteratorNotIndirectlyWritable>);
+static_assert(!HasRemoveIfIt<int*, int*, OutputIteratorNotInputOrOutputIterator>);
+
+template <class Range, class OutIter = int*>
+concept HasReplaceCopyR =
+    requires(Range range, OutIter result) { std::ranges::replace_copy_if(range, result, FalsePredicate{}, 0); };
+
+static_assert(HasReplaceCopyR<UncheckedRange<int*>>);
+static_assert(!HasReplaceCopyR<InputRangeNotDerivedFrom>);
+static_assert(!HasReplaceCopyR<InputRangeNotIndirectlyReadable>);
+static_assert(!HasReplaceCopyR<InputRangeNotInputOrOutputIterator>);
+static_assert(!HasReplaceCopyR<InputRangeNotSentinelSemiregular>);
+static_assert(!HasReplaceCopyR<InputRangeNotSentinelEqualityComparableWith>);
+static_assert(!HasReplaceCopyR<UncheckedRange<int*>, OutputIteratorNotIndirectlyWritable>);
+static_assert(!HasReplaceCopyR<UncheckedRange<int*>, OutputIteratorNotInputOrOutputIterator>);
+
+template <int N>
+struct Data {
+  std::array<int, N> input;
+  int cutoff;
+  int new_value;
+  std::array<int, N> expected;
+};
+
+template <class InIter, class Sent, class OutIter, int N>
+constexpr void test(Data<N> d) {
+  { // iterator overload
+    std::array<int, N> output;
+
+    auto first  = InIter(d.input.data());
+    auto last   = Sent(InIter(d.input.data() + d.input.size()));
+    auto result = OutIter(output.data());
+
+    auto pred = [&](int i) { return i < d.cutoff; };
+
+    std::same_as<std::ranges::in_out_result<InIter, OutIter>> decltype(auto) ret =
+        std::ranges::replace_copy_if(std::move(first), std::move(last), std::move(result), pred, d.new_value);
+    assert(base(ret.in) == d.input.data() + d.input.size());
+    assert(base(ret.out) == output.data() + output.size());
+    assert(d.expected == output);
+  }
+  { // range overload
+    std::array<int, N> output;
+
+    auto range  = std::ranges::subrange(InIter(d.input.data()), Sent(InIter(d.input.data() + d.input.size())));
+    auto result = OutIter(output.data());
+
+    auto pred = [&](int i) { return i < d.cutoff; };
+
+    std::same_as<std::ranges::in_out_result<InIter, OutIter>> decltype(auto) ret =
+        std::ranges::replace_copy_if(range, result, pred, d.new_value);
+    assert(base(ret.in) == d.input.data() + d.input.size());
+    assert(base(ret.out) == output.data() + output.size());
+    assert(d.expected == output);
+  }
+}
+
+template <class InIter, class Sent, class OutIter>
+constexpr void tests() {
+  // simple test
+  test<InIter, Sent, OutIter, 4>({.input = {1, 2, 3, 4}, .cutoff = 2, .new_value = 5, .expected = {5, 2, 3, 4}});
+  // empty range
+  test<InIter, Sent, OutIter, 0>({.input = {}, .cutoff = 2, .new_value = 5, .expected = {}});
+  // all elements match
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .cutoff = 2, .new_value = 2, .expected = {2, 2, 2, 2}});
+  // no element matches
+  test<InIter, Sent, OutIter, 4>({.input = {1, 1, 1, 1}, .cutoff = 1, .new_value = 3, .expected = {1, 1, 1, 1}});
+  // more elements
+  test<InIter, Sent, OutIter, 7>(
+      {.input = {1, 2, 3, 4, 5, 6, 7}, .cutoff = 3, .new_value = 3, .expected = {3, 3, 3, 4, 5, 6, 7}});
+  // single element - match
+  test<InIter, Sent, OutIter, 1>({.input = {1}, .cutoff = 2, .new_value = 5, .expected = {5}});
+  // single element - no match
+  test<InIter, Sent, OutIter, 1>({.input = {2}, .cutoff = 2, .new_value = 5, .expected = {2}});
+}
+
+template <class InIter, class Sent>
+constexpr void test_output_iterators() {
+  tests<InIter, Sent, cpp17_output_iterator<int*>>();
+  tests<InIter, Sent, forward_iterator<int*>>();
+  tests<InIter, Sent, bidirectional_iterator<int*>>();
+  tests<InIter, Sent, random_access_iterator<int*>>();
+  tests<InIter, Sent, contiguous_iterator<int*>>();
+  tests<InIter, Sent, int*>();
+}
+
+template <class InIter>
+constexpr void test_sentinels() {
+  test_output_iterators<InIter, InIter>();
+  test_output_iterators<InIter, sentinel_wrapper<InIter>>();
+  test_output_iterators<InIter, sized_sentinel<InIter>>();
+}
 
 constexpr bool test() {
-  // TODO: main tests.
-  // TODO: A custom comparator works.
-  // TODO: A custom projection works.
+  test_output_iterators<cpp17_input_iterator<int*>, sentinel_wrapper<cpp17_input_iterator<int*>>>();
+  test_output_iterators<cpp20_input_iterator<int*>, sentinel_wrapper<cpp20_input_iterator<int*>>>();
+  test_sentinels<forward_iterator<int*>>();
+  test_sentinels<bidirectional_iterator<int*>>();
+  test_sentinels<random_access_iterator<int*>>();
+  test_sentinels<contiguous_iterator<int*>>();
+  test_sentinels<int*>();
+  test_sentinels<const int*>();
+
+  { // check that a custom projection works
+    struct S {
+      int i;
+    };
+    { // iterator overload
+      S a[] = {{1}, {2}, {3}, {4}};
+      S b[4];
+      auto ret = std::ranges::replace_copy_if(std::begin(a), std::end(a), std::begin(b), FalsePredicate{}, S{2}, &S::i);
+      assert(ret.in == std::end(a));
+      assert(ret.out == std::end(b));
+      assert(std::ranges::equal(a, b, {}, &S::i, &S::i));
+    }
+    { // range overload
+      S a[] = {{1}, {2}, {3}, {4}};
+      S b[4];
+      auto ret = std::ranges::replace_copy_if(a, std::begin(b), FalsePredicate{}, S{2}, &S::i);
+      assert(ret.in == std::end(a));
+      assert(ret.out == std::end(b));
+      assert(std::ranges::equal(a, b, {}, &S::i, &S::i));
+    }
+  }
+
+  {   // check complexity requirements
+    { // iterator overload
+      int proj_count = 0;
+      auto proj      = [&](int i) {
+        ++proj_count;
+        return i;
+      };
+      int a[] = {1, 2, 3, 4};
+      int b[4];
+      std::ranges::replace_copy_if(std::begin(a), std::end(a), std::begin(b), FalsePredicate{}, 0, proj);
+      assert(proj_count == 4);
+    }
+    { // range overload
+      int proj_count = 0;
+      auto proj      = [&](int i) {
+        ++proj_count;
+        return i;
+      };
+      int a[] = {1, 2, 3, 4};
+      int b[4];
+      std::ranges::replace_copy_if(a, std::begin(b), FalsePredicate{}, 0, proj);
+      assert(proj_count == 4);
+    }
+  }
+
+  { // using different types the old and new value works
+    struct S {
+      constexpr operator int() const { return 1; }
+    };
+    {
+      int a[] = {0, 0, 2, 3};
+      int b[4];
+      std::ranges::replace_copy_if(std::begin(a), std::end(a), std::begin(b), [](int i) { return i < 2; }, S{});
+      assert(std::ranges::equal(b, std::array{1, 1, 2, 3}));
+    }
+    {
+      int a[] = {0, 0, 2, 3};
+      int b[4];
+      std::ranges::replace_copy_if(a, std::begin(b), [](int i) { return i < 2; }, S{});
+      assert(std::ranges::equal(b, std::array{1, 1, 2, 3}));
+    }
+  }
 
   return true;
 }
diff --git a/libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.compile.pass.cpp b/libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.compile.pass.cpp
--- a/libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.compile.pass.cpp
+++ b/libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.compile.pass.cpp
@@ -141,8 +141,8 @@
   //std::ranges::replace_if(in.begin(), in.end(), unary_pred, x);
   //std::ranges::replace_if(in, unary_pred, x);
   // replace_copy_if
-  //std::ranges::replace_copy_if(in.begin(), in.end(), out, unary_pred, x);
-  //std::ranges::replace_copy_if(in, out, unary_pred, x);
+  std::ranges::replace_copy_if(in.begin(), in.end(), out, unary_pred, x);
+  std::ranges::replace_copy_if(in, out, unary_pred, x);
 
   //in_out_pred(std::ranges::unique_copy, in, out, binary_pred);
   // partition_copy
diff --git a/libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.compile.pass.cpp b/libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.compile.pass.cpp
--- a/libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.compile.pass.cpp
+++ b/libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.compile.pass.cpp
@@ -176,11 +176,11 @@
   std::ranges::replace_if(in.begin(), in.end(), &Foo::unary_pred, a, &Bar::val);
   std::ranges::replace_if(in, &Foo::unary_pred, a, &Bar::val);
   // replace_copy
-  //std::ranges::replace_copy(in.begin(), in.end(), out, x, a, &Bar::val);
-  //std::ranges::replace_copy(in, out, x, a, &Bar::val);
+  std::ranges::replace_copy(in.begin(), in.end(), out, x, a, &Bar::val);
+  std::ranges::replace_copy(in, out, x, a, &Bar::val);
   // replace_copy_if
-  //std::ranges::replace_copy_if(in.begin(), in.end(), out, pred, a, &Bar::val);
-  //std::ranges::replace_copy_if(in, out, pred, a, &Bar::val);
+  std::ranges::replace_copy_if(in.begin(), in.end(), out, &Foo::unary_pred, a, &Bar::val);
+  std::ranges::replace_copy_if(in, out, &Foo::unary_pred, a, &Bar::val);
   // `swap_ranges` has neither a projection nor a predicate.
   // `reverse_copy` has neither a projection nor a predicate.
   // `rotate_copy` has neither a projection nor a predicate.