diff --git a/clang/lib/Basic/Targets/OSTargets.h b/clang/lib/Basic/Targets/OSTargets.h
--- a/clang/lib/Basic/Targets/OSTargets.h
+++ b/clang/lib/Basic/Targets/OSTargets.h
@@ -722,6 +722,11 @@
     if (Opts.CPlusPlus && Opts.WChar) {
       Builder.defineMacro("_WCHAR_T");
     }
+
+    // The AIX library implementation of C11 atomics is not yet complete.
+    if (Opts.C11 || Opts.C17 || Opts.C2x) {
+      Builder.defineMacro("__STDC_NO_ATOMICS__");
+    }
   }
 
 public:
diff --git a/clang/test/Preprocessor/aix-nocatomics.c b/clang/test/Preprocessor/aix-nocatomics.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/aix-nocatomics.c
@@ -0,0 +1,19 @@
+// The AIX library implementation of C11 atomics is incomplete, so test that the
+// appropriate macro is set.
+
+// 32-bit cases.
+// RUN: %clang_cc1 -E -dM -triple=powerpc-ibm-aix7.1.0.0 -std=c11 < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -E -dM -triple=powerpc-ibm-aix7.1.0.0 -std=c17 < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -E -dM -triple=powerpc-ibm-aix7.1.0.0 -std=c2x < /dev/null | FileCheck -match-full-lines %s
+
+// 64-bit cases.
+// RUN: %clang_cc1 -E -dM -triple=powerpc64-ibm-aix7.1.0.0 -std=c11 < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -E -dM -triple=powerpc64-ibm-aix7.1.0.0 -std=c17 < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -E -dM -triple=powerpc64-ibm-aix7.1.0.0 -std=c2x < /dev/null | FileCheck -match-full-lines %s
+
+// CHECK:#define __STDC_NO_ATOMICS__ 1
+
+// Check that it's not set for C before 11.
+// RUN: %clang_cc1 -E -dM -triple=powerpc64-ibm-aix7.1.0.0 -std=c99 < /dev/null | FileCheck -match-full-lines -check-prefix C99 %s
+
+// C99-NOT:#define __STDC_NO_ATOMICS__ 1