diff --git a/libcxx/include/complex b/libcxx/include/complex
index e9baec04d9465..d8ec3d95c10ed 100644
--- a/libcxx/include/complex
+++ b/libcxx/include/complex
@@ -260,6 +260,7 @@ template<class T> complex<T> tanh (const complex<T>&);
 #  include <__cxx03/complex>
 #else
 #  include <__config>
+#  include <__cstddef/size_t.h>
 #  include <__fwd/complex.h>
 #  include <__fwd/tuple.h>
 #  include <__tuple/tuple_element.h>
diff --git a/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_element.compile.pass.cpp b/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_element.compile.pass.cpp
index 50c0e0eff1eda..3a220578a074c 100644
--- a/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_element.compile.pass.cpp
+++ b/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_element.compile.pass.cpp
@@ -16,7 +16,7 @@
 #include <complex>
 #include <concepts>
 
-template <size_t I, typename C>
+template <std::size_t I, typename C>
 concept HasTupleElement = requires { std::tuple_element<I, C>{}; };
 
 struct SomeObject {};
diff --git a/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_size.compile.pass.cpp b/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_size.compile.pass.cpp
index 28dcbe3fc5ecd..1091c6345598e 100644
--- a/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_size.compile.pass.cpp
+++ b/libcxx/test/std/numerics/complex.number/complex.tuple/tuple_size.compile.pass.cpp
@@ -28,7 +28,7 @@ void test() {
   using C = std::complex<T>;
 
   static_assert(HasTupleSize<C>);
-  static_assert(std::same_as<typename std::tuple_size<C>::value_type, size_t>);
+  static_assert(std::same_as<typename std::tuple_size<C>::value_type, std::size_t>);
   static_assert(std::tuple_size<C>() == 2);
 }