Index: lib/Basic/Targets.cpp
===================================================================
--- lib/Basic/Targets.cpp
+++ lib/Basic/Targets.cpp
@@ -569,19 +569,27 @@
         Triple.getVendor() != llvm::Triple::UnknownVendor ||
         !Triple.isOSBinFormatWasm())
       return nullptr;
-    if (Triple.getOS() != llvm::Triple::UnknownOS &&
-        Triple.getOS() != llvm::Triple::WASI)
-      return nullptr;
-    return new WebAssemblyOSTargetInfo<WebAssembly32TargetInfo>(Triple, Opts);
+    switch (Triple.getOS()) {
+      case llvm::Triple::WASI:
+        return new WASITargetInfo<WebAssembly32TargetInfo>(Triple, Opts);
+      case llvm::Triple::UnknownOS:
+        return new WebAssemblyOSTargetInfo<WebAssembly32TargetInfo>(Triple, Opts);
+      default:
+        return nullptr;
+    }
   case llvm::Triple::wasm64:
     if (Triple.getSubArch() != llvm::Triple::NoSubArch ||
         Triple.getVendor() != llvm::Triple::UnknownVendor ||
         !Triple.isOSBinFormatWasm())
       return nullptr;
-    if (Triple.getOS() != llvm::Triple::UnknownOS &&
-        Triple.getOS() != llvm::Triple::WASI)
-      return nullptr;
-    return new WebAssemblyOSTargetInfo<WebAssembly64TargetInfo>(Triple, Opts);
+    switch (Triple.getOS()) {
+      case llvm::Triple::WASI:
+        return new WASITargetInfo<WebAssembly64TargetInfo>(Triple, Opts);
+      case llvm::Triple::UnknownOS:
+        return new WebAssemblyOSTargetInfo<WebAssembly64TargetInfo>(Triple, Opts);
+      default:
+        return nullptr;
+    }
 
   case llvm::Triple::renderscript32:
     return new LinuxTargetInfo<RenderScript32TargetInfo>(Triple, Opts);
Index: lib/Basic/Targets/OSTargets.h
===================================================================
--- lib/Basic/Targets/OSTargets.h
+++ lib/Basic/Targets/OSTargets.h
@@ -764,7 +764,7 @@
 class LLVM_LIBRARY_VISIBILITY WebAssemblyOSTargetInfo
     : public OSTargetInfo<Target> {
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
-                    MacroBuilder &Builder) const final {
+                    MacroBuilder &Builder) const {
     // A common platform macro.
     if (Opts.POSIXThreads)
       Builder.defineMacro("_REENTRANT");
@@ -782,6 +782,21 @@
   }
 };
 
+// WASI target
+template <typename Target>
+class LLVM_LIBRARY_VISIBILITY WASITargetInfo
+    : public WebAssemblyOSTargetInfo<Target> {
+  void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
+                    MacroBuilder &Builder) const final {
+    WebAssemblyOSTargetInfo::getOSDefines(Opts, Triple, Builder);
+    Builder.defineMacro("__wasi__");
+  }
+
+public:
+  explicit WASITargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)
+      : WebAssemblyOSTargetInfo<Target>(Triple, Opts) {}
+};
+
 } // namespace targets
 } // namespace clang
 #endif // LLVM_CLANG_LIB_BASIC_TARGETS_OSTARGETS_H
Index: test/Preprocessor/init.c
===================================================================
--- test/Preprocessor/init.c
+++ test/Preprocessor/init.c
@@ -9111,9 +9111,15 @@
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=wasm32-unknown-unknown \
 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefix=WEBASSEMBLY32 %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -triple=wasm32-unknown-wasi \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY32,WEBASSEMBLY-WASI %s
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=wasm64-unknown-unknown \
 // RUN:   < /dev/null \
 // RUN:   | FileCheck -match-full-lines -check-prefix=WEBASSEMBLY64 %s
+// RUN: %clang_cc1 -E -dM -ffreestanding -triple=wasm64-unknown-wasi \
+// RUN:   < /dev/null \
+// RUN:   | FileCheck -match-full-lines -check-prefixes=WEBASSEMBLY,WEBASSEMBLY64,WEBASSEMBLY-WASI %s
 //
 // WEBASSEMBLY32:#define _ILP32 1
 // WEBASSEMBLY32-NOT:#define _LP64
@@ -9428,6 +9431,7 @@
 // WEBASSEMBLY-NEXT:#define __clang_patchlevel__ {{.*}}
 // WEBASSEMBLY-NEXT:#define __clang_version__ "{{.*}}"
 // WEBASSEMBLY-NEXT:#define __llvm__ 1
+// WEBASSEMBLY-WASI-NEXT:#define __wasi__ 1
 // WEBASSEMBLY-NOT:#define __wasm_simd128__
 // WEBASSEMBLY-NOT:#define __wasm_simd256__
 // WEBASSEMBLY-NOT:#define __wasm_simd512__