diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -58,6 +58,11 @@
   release being diagnosed against). These new groups are automatically implied
   when passing ``-Wc++N-extensions``. Resolves PR33518.
 
+- Support ``-Wdeclaration-after-statement`` with C99 and later standards, and
+  not just C89, matching GCC's behaviour. A notable usecase is supporting style
+  guides that forbid mixing declarations and code, but want to move to newer C
+  standards.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -413,7 +413,10 @@
   // If we're in C mode, check that we don't have any decls after stmts.  If
   // so, emit an extension diagnostic in C89 and potentially a warning in later
   // versions.
-  if (!getLangOpts().CPlusPlus) {
+  const unsigned MixedDeclsCodeID = getLangOpts().C99
+                                        ? diag::warn_mixed_decls_code
+                                        : diag::ext_mixed_decls_code;
+  if (!getLangOpts().CPlusPlus && !Diags.isIgnored(MixedDeclsCodeID, L)) {
     // Note that __extension__ can be around a decl.
     unsigned i = 0;
     // Skip over all declarations.
@@ -426,8 +429,7 @@
 
     if (i != NumElts) {
       Decl *D = *cast<DeclStmt>(Elts[i])->decl_begin();
-      Diag(D->getLocation(), !getLangOpts().C99 ? diag::ext_mixed_decls_code
-                                                : diag::warn_mixed_decls_code);
+      Diag(D->getLocation(), MixedDeclsCodeID);
     }
   }
 
diff --git a/clang/test/Sema/warn-mixed-decls.c b/clang/test/Sema/warn-mixed-decls.c
--- a/clang/test/Sema/warn-mixed-decls.c
+++ b/clang/test/Sema/warn-mixed-decls.c
@@ -1,13 +1,23 @@
 /* RUN: %clang_cc1 -fsyntax-only -verify -std=c89 -pedantic %s
  */
+/* RUN: %clang_cc1 -fsyntax-only -verify -std=c89 -Wdeclaration-after-statement %s
+ */
 /* RUN: %clang_cc1 -fsyntax-only -verify -std=c99 -Wdeclaration-after-statement %s
  */
+/* RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -Wdeclaration-after-statement %s
+ */
 
 /* Should not emit diagnostic when not pedantic, not enabled or in C++ Code*/
 /* RUN: %clang_cc1 -fsyntax-only -verify=none -std=c89 %s
  */
 /* RUN: %clang_cc1 -fsyntax-only -verify=none -std=c99 %s
  */
+/* RUN: %clang_cc1 -fsyntax-only -verify=none -std=c89 -Wall %s
+ */
+/* RUN: %clang_cc1 -fsyntax-only -verify=none -std=c99 -Wall -pedantic %s
+ */
+/* RUN: %clang_cc1 -fsyntax-only -verify=none -std=c11 -Wall -pedantic %s
+ */
 /* RUN: %clang_cc1 -fsyntax-only -verify=none -x c++ %s
  */
 /* RUN: %clang_cc1 -fsyntax-only -verify=none -x c++ -Wdeclaration-after-statement %s