diff --git a/ChangeLog b/ChangeLog
index 9343f91de3..a32ce789d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2026-10-01  Bruno Haible  <bruno@clisp.org>
+
+	tzsanitize: Fix test failures on MSVC.
+	* lib/tzsanitize.c: Include <windows.h>.
+	(tzsanitize): Except on mingw, sanitize also the environment variable
+	in the process environment block.
+
 2026-10-01  Bruno Haible  <bruno@clisp.org>
 
 	tzsanitize: New module.
diff --git a/lib/tzsanitize.c b/lib/tzsanitize.c
index 291bcd6170..dd4f31173f 100644
--- a/lib/tzsanitize.c
+++ b/lib/tzsanitize.c
@@ -25,6 +25,11 @@
 #include <string.h>
 #include <wchar.h>
 
+#if !defined __MINGW32__
+# define WIN32_LEAN_AND_MEAN  /* avoid including junk */
+# include <windows.h>
+#endif
+
 void
 tzsanitize (void)
 {
@@ -51,5 +56,28 @@ tzsanitize (void)
             if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
               ws[0] = L'$';
           }
+#if !defined __MINGW32__
+      SetEnvironmentVariable ("TZ", NULL);
+#endif
     }
+#if !defined __MINGW32__
+  /* On MSVC, we also need to sanitize the environment variables stored in the
+     process's environment block.
+     We don't lose much by assuming that the environment variable's value
+     is not too large.  */
+  else
+    {
+      /* Documentation:
+         <https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getenvironmentvariable>  */
+      char tz_buf[256];
+      DWORD tz_len = GetEnvironmentVariable ("TZ", tz_buf, sizeof (tz_buf));
+      if (tz_len > 0 && tz_len < sizeof (tz_buf)
+          && strchr (tz_buf, '/') != NULL)
+        {
+          /* Documentation:
+             <https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-setenvironmentvariable>  */
+          SetEnvironmentVariable ("TZ", NULL);
+        }
+    }
+#endif
 }