diff --git a/clang/lib/Basic/Targets/WebAssembly.h b/clang/lib/Basic/Targets/WebAssembly.h
--- a/clang/lib/Basic/Targets/WebAssembly.h
+++ b/clang/lib/Basic/Targets/WebAssembly.h
@@ -138,6 +138,8 @@
   bool hasExtIntType() const override { return true; }
 
   bool hasProtectedVisibility() const override { return false; }
+
+  void adjust(LangOptions &Opts) override;
 };
 
 class LLVM_LIBRARY_VISIBILITY WebAssembly32TargetInfo
diff --git a/clang/lib/Basic/Targets/WebAssembly.cpp b/clang/lib/Basic/Targets/WebAssembly.cpp
--- a/clang/lib/Basic/Targets/WebAssembly.cpp
+++ b/clang/lib/Basic/Targets/WebAssembly.cpp
@@ -253,6 +253,15 @@
                                              Builtin::FirstTSBuiltin);
 }
 
+void WebAssemblyTargetInfo::adjust(LangOptions &Opts) {
+  // If the Atomics feature isn't available, turn off POSIXThreads and
+  // ThreadModel, so that we don't predefine _REENTRANT or __STDCPP_THREADS__.
+  if (!HasAtomics) {
+    Opts.POSIXThreads = false;
+    Opts.setThreadModel(LangOptions::ThreadModelKind::Single);
+  }
+}
+
 void WebAssembly32TargetInfo::getTargetDefines(const LangOptions &Opts,
                                                MacroBuilder &Builder) const {
   WebAssemblyTargetInfo::getTargetDefines(Opts, Builder);
diff --git a/clang/test/Preprocessor/init.c b/clang/test/Preprocessor/init.c
--- a/clang/test/Preprocessor/init.c
+++ b/clang/test/Preprocessor/init.c
@@ -1479,6 +1479,12 @@
 // RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm64-wasi \
 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY64,WEBASSEMBLY-WASI %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm32-unknown-unknown -x c++ \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY-CXX %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -fgnuc-version=4.2.1 -triple=wasm32-unknown-unknown -x c++ -pthread -target-feature +atomics \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY-CXX-ATOMICS %s
 //
 // WEBASSEMBLY32:#define _ILP32 1
 // WEBASSEMBLY32-NOT:#define _LP64
@@ -1847,6 +1853,10 @@
 // WEBASSEMBLY64-NEXT:#define __wasm64 1
 // WEBASSEMBLY64-NEXT:#define __wasm64__ 1
 // WEBASSEMBLY-NEXT:#define __wasm__ 1
+// WEBASSEMBLY-CXX-NOT:_REENTRANT
+// WEBASSEMBLY-CXX-NOT:__STDCPP_THREADS__
+// WEBASSEMBLY-CXX-ATOMICS:#define _REENTRANT 1
+// WEBASSEMBLY-CXX-ATOMICS:#define __STDCPP_THREADS__ 1
 
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple i686-windows-cygnus < /dev/null | FileCheck -match-full-lines -check-prefix CYGWIN-X32 %s
 // CYGWIN-X32: #define __USER_LABEL_PREFIX__ _