diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 4672810a21a2..9ec146a6798e 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -193,11 +193,11 @@ template <class ForwardIterator1, class ForwardIterator2>
     iter_swap(ForwardIterator1 a, ForwardIterator2 b);
 
 template <class InputIterator, class OutputIterator, class UnaryOperation>
-    OutputIterator
+    constexpr OutputIterator      // constexpr in C++20
     transform(InputIterator first, InputIterator last, OutputIterator result, UnaryOperation op);
 
 template <class InputIterator1, class InputIterator2, class OutputIterator, class BinaryOperation>
-    OutputIterator
+    constexpr OutputIterator      // constexpr in C++20
     transform(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2,
               OutputIterator result, BinaryOperation binary_op);
 
@@ -1946,7 +1946,7 @@ move_backward(_BidirectionalIterator1 __first, _BidirectionalIterator1 __last,
 // transform
 
 template <class _InputIterator, class _OutputIterator, class _UnaryOperation>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 transform(_InputIterator __first, _InputIterator __last, _OutputIterator __result, _UnaryOperation __op)
 {
@@ -1956,7 +1956,7 @@ transform(_InputIterator __first, _InputIterator __last, _OutputIterator __resul
 }
 
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _BinaryOperation>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 transform(_InputIterator1 __first1, _InputIterator1 __last1, _InputIterator2 __first2,
           _OutputIterator __result, _BinaryOperation __binary_op)
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/binary_transform.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/binary_transform.pass.cpp
index 8491ea59eff6..27d3a968a768 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/binary_transform.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/binary_transform.pass.cpp
@@ -12,15 +12,34 @@
 // template<InputIterator InIter1, InputIterator InIter2, class OutIter,
 //          Callable<auto, const InIter1::value_type&, const InIter2::value_type&> BinaryOp>
 //   requires OutputIterator<OutIter, BinaryOp::result_type> && CopyConstructible<BinaryOp>
-// OutIter
+// constexpr OutIter      // constexpr after C++17
 // transform(InIter1 first1, InIter1 last1, InIter2 first2, OutIter result, BinaryOp binary_op);
 
 #include <algorithm>
 #include <functional>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    const int ia[] = {1, 3, 6, 7};
+    const int ib[] = {2, 4, 7, 8};
+          int ic[] = {0, 0, 0, 0, 0}; // one bigger
+    const int expected[] = {3, 7, 13, 15};
+    
+    auto it = std::transform(std::begin(ia), std::end(ia), 
+                             std::begin(ib), std::begin(ic), std::plus<int>());
+    
+    return it == (std::begin(ic) + std::size(ia))
+        && *it == 0 // don't overwrite the last value in the output array
+        && std::equal(std::begin(expected), std::end(expected), std::begin(ic), it)
+        ;
+    }
+#endif
+
+
 template<class InIter1, class InIter2, class OutIter>
 void
 test()
@@ -214,4 +233,8 @@ int main()
     test<const int*, const int*, bidirectional_iterator<int*> >();
     test<const int*, const int*, random_access_iterator<int*> >();
     test<const int*, const int*, int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }
diff --git a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/unary_transform.pass.cpp b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/unary_transform.pass.cpp
index 6c5e621e4b20..b6f9cbad00fe 100644
--- a/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/unary_transform.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.modifying.operations/alg.transform/unary_transform.pass.cpp
@@ -12,16 +12,34 @@
 // template<InputIterator InIter, class OutIter,
 //          Callable<auto, const InIter::value_type&> Op>
 //   requires OutputIterator<OutIter, Op::result_type> && CopyConstructible<Op>
-//   OutIter
+// constexpr OutIter      // constexpr after C++17
 //   transform(InIter first, InIter last, OutIter result, Op op);
 
 #include <algorithm>
 #include <functional>
 #include <cassert>
 
+#include "test_macros.h"
 #include "test_iterators.h"
 
-int plusOne(int v) { return v + 1; }
+TEST_CONSTEXPR int plusOne(int v) { return v + 1; }
+
+
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+    int ia[] = {1, 3, 6, 7};
+    int ib[] = {0, 0, 0, 0, 0}; // one bigger
+    const int expected[] = {2, 4, 7, 8};
+
+    auto it = std::transform(std::begin(ia), std::end(ia), std::begin(ib), plusOne);
+    
+    return it == (std::begin(ib) + std::size(ia))
+        && *it == 0 // don't overwrite the last value in the output array
+        && std::equal(std::begin(ib), it, std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
+
 
 template <class InIter, class OutIter>
 void
@@ -76,4 +94,8 @@ int main()
     test<const int*, bidirectional_iterator<int*> >();
     test<const int*, random_access_iterator<int*> >();
     test<const int*, int*>();
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }