diff --git a/libcxx/include/__random/binomial_distribution.h b/libcxx/include/__random/binomial_distribution.h
index e8774bb8d67ee..54f321af63d50 100644
--- a/libcxx/include/__random/binomial_distribution.h
+++ b/libcxx/include/__random/binomial_distribution.h
@@ -97,10 +97,6 @@ class _LIBCPP_TEMPLATE_VIS binomial_distribution {
   }
 };
 
-#ifndef _LIBCPP_MSVCRT_LIKE
-extern "C" double lgamma_r(double, int*);
-#endif
-
 inline _LIBCPP_HIDE_FROM_ABI double __libcpp_lgamma(double __d) {
 #if defined(_LIBCPP_MSVCRT_LIKE)
   return lgamma(__d);
diff --git a/libcxx/include/math.h b/libcxx/include/math.h
index 05989734c26c6..0e8bf8f1f65a2 100644
--- a/libcxx/include/math.h
+++ b/libcxx/include/math.h
@@ -297,6 +297,13 @@ long double    truncl(long double x);
 #    pragma GCC system_header
 #  endif
 
+// This is required to define functions like lgamma_r on Apple platforms.
+// These functions are not required by the Standard but our implementation
+// uses them.
+#  if defined(__APPLE__) && !defined(_REENTRANT)
+#    define _REENTRANT
+#  endif
+
 #  if __has_include_next(<math.h>)
 #    include_next <math.h>
 #  endif