diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -11588,7 +11588,9 @@
 def err_hlsl_numthreads_invalid : Error<"total number of threads cannot exceed %0">;
 def err_hlsl_attribute_param_mismatch : Error<"%0 attribute parameters do not match the previous declaration">;
 
-// Layout randomization warning.
+// Layout randomization diagnostics.
+def err_non_designated_init_used : Error<
+  "a randomized struct can only be initialized with a designated initializer">;
 def err_cast_from_randomized_struct : Error<
   "casting from randomized structure pointer type %0 to %1">;
 } // end of sema component.
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2176,6 +2176,27 @@
       break;
     }
 
+    // Check if this is an initializer of forms:
+    //
+    //   struct foo f = {};
+    //   struct foo g = {0};
+    //
+    // These are okay for randomized structures. [C99 6.7.8p19]
+    auto IsZeroInitializer = [&](const Expr *I) {
+      if (IList->getNumInits() == 1)
+        if (const auto *IL = dyn_cast<IntegerLiteral>(I))
+          return IL->getValue().isZero();
+      return false;
+    };
+
+    // Don't allow non-designated initializers on randomized structures.
+    if (RD->isRandomized() && !IsZeroInitializer(Init)) {
+      if (!VerifyOnly)
+        SemaRef.Diag(InitLoc, diag::err_non_designated_init_used);
+      hadError = true;
+      break;
+    }
+
     // We've already initialized a member of a union. We're done.
     if (InitializedSomething && DeclType->isUnionType())
       break;
diff --git a/clang/test/Sema/init-randomized-struct.c b/clang/test/Sema/init-randomized-struct.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/init-randomized-struct.c
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -frandomize-layout-seed=1234567890abcdef
+
+typedef void (*func_ptr)();
+
+void foo(void);
+void bar(void);
+void baz(void);
+void gaz(void);
+
+struct test {
+  func_ptr a;
+  func_ptr b;
+  func_ptr c;
+  func_ptr d;
+  func_ptr e;
+  func_ptr f;
+  func_ptr g;
+} __attribute__((randomize_layout));
+
+struct test t1 = {}; // This should be fine per WG14 N2900 (in C23) + our extension handling of it in earlier modes
+struct test t2 = {0}; // This should also be fine per C99 6.7.8p19
+
+struct test t3 = { .a = foo, bar, baz }; // expected-error {{a randomized struct can only be initialized with a designated initializer}}
+
+struct other_test {
+  func_ptr a;
+  func_ptr b[3];
+  func_ptr c;
+} __attribute__((randomize_layout));
+
+struct other_test t4 = { .a = foo, .b[0] = foo }; // Ok
+struct other_test t5 = { .a = foo, .b[0] = foo, bar, baz }; // Ok
+struct other_test t7 = { .a = foo, .b = { foo, bar, baz } }; // Ok
+struct other_test t6 = { .a = foo, .b[0] = foo, bar, baz, gaz }; // expected-error {{a randomized struct can only be initialized with a designated initializer}}