diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index bf4f415c15db..44830a6c9720 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,7 @@
 2017-01-06  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/c++config (_GLIBCXX_ASSERTIONS): Avoid redefinition.
+
 	PR libstdc++/78991
 	* include/bits/predefined_ops.h (_Iter_comp_iter, _Iter_comp_val)
 	(_Val_comp_iter, _Iter_equals_val, _Iter_pred, _Iter_comp_to_val)
diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index b2eea3ca36a9..0cc1865294c2 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -423,7 +423,7 @@ namespace std
 #endif
 
 // Debug Mode implies checking assertions.
-#ifdef _GLIBCXX_DEBUG
+#if defined(_GLIBCXX_DEBUG) && !defined(_GLIBCXX_ASSERTIONS)
 # define _GLIBCXX_ASSERTIONS 1
 #endif