diff --git a/clang/lib/Driver/ToolChains/WebAssembly.cpp b/clang/lib/Driver/ToolChains/WebAssembly.cpp
--- a/clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ b/clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -90,6 +90,28 @@
   CmdArgs.push_back(Output.getFilename());
 
   C.addCommand(llvm::make_unique<Command>(JA, *this, Linker, CmdArgs, Inputs));
+
+  // When optimizing, if WASM_OPT is set, run wasm-opt.
+  if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
+    if (const char *WasmOptPath = getenv("WASM_OPT")) {
+      StringRef OOpt = "s";
+      if (A->getOption().matches(options::OPT_O4) ||
+          A->getOption().matches(options::OPT_Ofast))
+        OOpt = "4";
+      else if (A->getOption().matches(options::OPT_O0))
+        OOpt = "0";
+      else if (A->getOption().matches(options::OPT_O))
+        OOpt = A->getValue();
+
+      const char *WasmOpt = Args.MakeArgString(WasmOptPath);
+      ArgStringList CmdArgs;
+      CmdArgs.push_back(Output.getFilename());
+      CmdArgs.push_back(Args.MakeArgString(llvm::Twine("-O") + OOpt));
+      CmdArgs.push_back("-o");
+      CmdArgs.push_back(Output.getFilename());
+      C.addCommand(llvm::make_unique<Command>(JA, *this, WasmOpt, CmdArgs, Inputs));
+    }
+  }
 }
 
 WebAssembly::WebAssembly(const Driver &D, const llvm::Triple &Triple,
@@ -109,6 +131,11 @@
   } else {
     const std::string MultiarchTriple =
         getMultiarchTriple(getDriver(), Triple, getDriver().SysRoot);
+    if (D.isUsingLTO()) {
+      // For LTO, enable use of lto-enabled sysroot libraries too, if available.
+      getFilePaths().push_back(getDriver().SysRoot + "/lib/" + MultiarchTriple +
+                               "/llvm-lto");
+    }
     getFilePaths().push_back(getDriver().SysRoot + "/lib/" + MultiarchTriple);
   }
 }
diff --git a/clang/test/Driver/wasm-toolchain-lto.c b/clang/test/Driver/wasm-toolchain-lto.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Driver/wasm-toolchain-lto.c
@@ -0,0 +1,6 @@
+// A basic C link command-line with optimization with known OS and LTO enabled.
+
+// RUN: %clang -### -O2 -flto -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=LINK_OPT_KNOWN %s
+// LINK_OPT_KNOWN: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
+// LINK_OPT_KNOWN: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi/llvm-lto" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"