diff --git a/libstdc++-v3/src/c++17/fs_ops.cc b/libstdc++-v3/src/c++17/fs_ops.cc
index 1d3693af06c..321944d73f7 100644
--- a/libstdc++-v3/src/c++17/fs_ops.cc
+++ b/libstdc++-v3/src/c++17/fs_ops.cc
@@ -27,6 +27,10 @@
 # define NEED_DO_COPY_FILE
 # define NEED_DO_SPACE
 #endif
+#ifndef _GNU_SOURCE
+// Cygwin needs this for secure_getenv
+# define _GNU_SOURCE 1
+#endif
 
 #include <bits/largefile-config.h>
 #include <filesystem>
diff --git a/libstdc++-v3/src/filesystem/dir.cc b/libstdc++-v3/src/filesystem/dir.cc
index f1bf96aab50..d5b11f25297 100644
--- a/libstdc++-v3/src/filesystem/dir.cc
+++ b/libstdc++-v3/src/filesystem/dir.cc
@@ -25,6 +25,10 @@
 #ifndef _GLIBCXX_USE_CXX11_ABI
 # define _GLIBCXX_USE_CXX11_ABI 1
 #endif
+#ifndef _GNU_SOURCE
+// Cygwin needs this for secure_getenv
+# define _GNU_SOURCE 1
+#endif
 
 #include <bits/largefile-config.h>
 #include <experimental/filesystem>
diff --git a/libstdc++-v3/src/filesystem/ops.cc b/libstdc++-v3/src/filesystem/ops.cc
index 324c6afc7a9..cd98caf73f2 100644
--- a/libstdc++-v3/src/filesystem/ops.cc
+++ b/libstdc++-v3/src/filesystem/ops.cc
@@ -27,6 +27,10 @@
 # define NEED_DO_COPY_FILE
 # define NEED_DO_SPACE
 #endif
+#ifndef _GNU_SOURCE
+// Cygwin needs this for secure_getenv
+# define _GNU_SOURCE 1
+#endif
 
 #include <bits/largefile-config.h>
 #include <experimental/filesystem>