diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -693,6 +693,9 @@
 def warn_cxx98_compat_pp_line_too_big : Warning<
   "#line number greater than 32767 is incompatible with C++98">,
   InGroup<CXX98CompatPedantic>, DefaultIgnore;
+def ext_c2x_pp_directive : Extension<
+  "%select{#elifdef|#elifndef}0 is a C2x extension">,
+  InGroup<CPre2xCompatPedantic>;
 
 def err_pp_visibility_non_macro : Error<"no macro named %0">;
 
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -3255,6 +3255,17 @@
                                                  : PED_Elifndef;
   ++NumElse;
 
+  // Warn if using `#elifdef` & `#elifndef` in not C2x mode.
+  switch (DirKind) {
+    case PED_Elifdef:
+    case PED_Elifndef:
+      if (!getLangOpts().C2x)
+        Diag(ElifToken, diag::ext_c2x_pp_directive) << DirKind - 1;
+      break;
+    default:
+      break;
+  }
+
   // #elif directive in a non-skipping conditional... start skipping.
   // We don't care what the condition is, because we will always skip it (since
   // the block immediately before it was included).
diff --git a/clang/test/Preprocessor/ext-c2x-pp-directive.c b/clang/test/Preprocessor/ext-c2x-pp-directive.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/ext-c2x-pp-directive.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify -pedantic %s
+// RUN: not %clang_cc1 -std=c99 -fsyntax-only -verify %s
+// RUN: not %clang_cc1 -std=c2x -fsyntax-only -verify -pedantic %s
+// RUN: not %clang_cc1 -std=c2x -fsyntax-only -verify %s
+
+#if 1
+#elifndef FOO // expected-warning {{#elifndef is a C2x extension}}
+#endif
+
+#if 1
+#elifdef BAR // expected-warning {{#elifdef is a C2x extension}}
+#endif
+
+// expected-warning {{ISO C requires a translation unit to contain at least one declaration}}