diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -3337,6 +3337,8 @@
 
 def msimd128 : Flag<["-"], "msimd128">, Group<m_wasm_Features_Group>;
 def mno_simd128 : Flag<["-"], "mno-simd128">, Group<m_wasm_Features_Group>;
+def mrelaxed_simd : Flag<["-"], "mrelaxed-simd">, Group<m_wasm_Features_Group>;
+def mno_relaxed_simd : Flag<["-"], "mno-relaxed-simd">, Group<m_wasm_Features_Group>;
 def mnontrapping_fptoint : Flag<["-"], "mnontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def mno_nontrapping_fptoint : Flag<["-"], "mno-nontrapping-fptoint">, Group<m_wasm_Features_Group>;
 def msign_ext : Flag<["-"], "msign-ext">, Group<m_wasm_Features_Group>;
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
@@ -27,6 +27,7 @@
   enum SIMDEnum {
     NoSIMD,
     SIMD128,
+    RelaxedSIMD,
   } SIMDLevel = NoSIMD;
 
   bool HasNontrappingFPToInt = false;
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
@@ -46,6 +46,7 @@
 bool WebAssemblyTargetInfo::hasFeature(StringRef Feature) const {
   return llvm::StringSwitch<bool>(Feature)
       .Case("simd128", SIMDLevel >= SIMD128)
+      .Case("relaxed-simd", SIMDLevel >= RelaxedSIMD)
       .Case("nontrapping-fptoint", HasNontrappingFPToInt)
       .Case("sign-ext", HasSignExt)
       .Case("exception-handling", HasExceptionHandling)
@@ -72,6 +73,8 @@
   defineCPUMacros(Builder, "wasm", /*Tuning=*/false);
   if (SIMDLevel >= SIMD128)
     Builder.defineMacro("__wasm_simd128__");
+  if (SIMDLevel >= RelaxedSIMD)
+    Builder.defineMacro("__wasm_relaxed_simd__");
   if (HasNontrappingFPToInt)
     Builder.defineMacro("__wasm_nontrapping_fptoint__");
   if (HasSignExt)
@@ -96,6 +99,9 @@
                                          SIMDEnum Level, bool Enabled) {
   if (Enabled) {
     switch (Level) {
+    case RelaxedSIMD:
+      Features["relaxed-simd"] = true;
+      LLVM_FALLTHROUGH;
     case SIMD128:
       Features["simd128"] = true;
       LLVM_FALLTHROUGH;
@@ -109,6 +115,9 @@
   case NoSIMD:
   case SIMD128:
     Features["simd128"] = false;
+    LLVM_FALLTHROUGH;
+  case RelaxedSIMD:
+    Features["relaxed-simd"] = false;
     break;
   }
 }
@@ -118,6 +127,8 @@
                                               bool Enabled) const {
   if (Name == "simd128")
     setSIMDLevel(Features, SIMD128, Enabled);
+  else if (Name == "relaxed-simd")
+    setSIMDLevel(Features, RelaxedSIMD, Enabled);
   else
     Features[Name] = Enabled;
 }
@@ -149,6 +160,14 @@
       SIMDLevel = std::min(SIMDLevel, SIMDEnum(SIMD128 - 1));
       continue;
     }
+    if (Feature == "+relaxed-simd") {
+      SIMDLevel = std::max(SIMDLevel, RelaxedSIMD);
+      continue;
+    }
+    if (Feature == "-relaxed-simd") {
+      SIMDLevel = std::min(SIMDLevel, SIMDEnum(RelaxedSIMD - 1));
+      continue;
+    }
     if (Feature == "+nontrapping-fptoint") {
       HasNontrappingFPToInt = true;
       continue;
diff --git a/clang/test/Preprocessor/wasm-target-features.c b/clang/test/Preprocessor/wasm-target-features.c
--- a/clang/test/Preprocessor/wasm-target-features.c
+++ b/clang/test/Preprocessor/wasm-target-features.c
@@ -7,6 +7,15 @@
 //
 // SIMD128:#define __wasm_simd128__ 1{{$}}
 
+// RUN: %clang -E -dM %s -o - 2>&1 \
+// RUN:     -target wasm32-unknown-unknown -mrelaxed-simd \
+// RUN:   | FileCheck %s -check-prefix=RELAXED-SIMD
+// RUN: %clang -E -dM %s -o - 2>&1 \
+// RUN:     -target wasm64-unknown-unknown -mrelaxed-simd \
+// RUN:   | FileCheck %s -check-prefix=RELAXED-SIMD
+//
+// RELAXED-SIMD:#define __wasm_relaxed_simd__ 1{{$}}
+
 // RUN: %clang -E -dM %s -o - 2>&1 \
 // RUN:     -target wasm32-unknown-unknown -mnontrapping-fptoint \
 // RUN:   | FileCheck %s -check-prefix=NONTRAPPING-FPTOINT
diff --git a/llvm/lib/Target/WebAssembly/WebAssembly.td b/llvm/lib/Target/WebAssembly/WebAssembly.td
--- a/llvm/lib/Target/WebAssembly/WebAssembly.td
+++ b/llvm/lib/Target/WebAssembly/WebAssembly.td
@@ -25,6 +25,9 @@
 def FeatureSIMD128 : SubtargetFeature<"simd128", "SIMDLevel", "SIMD128",
                                       "Enable 128-bit SIMD">;
 
+def FeatureRelaxedSIMD : SubtargetFeature<"relaxed-simd", "SIMDLevel", "RelaxedSIMD",
+                                      "Enable relaxed-simd instructions">;
+
 def FeatureAtomics : SubtargetFeature<"atomics", "HasAtomics", "true",
                                       "Enable Atomics">;
 
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.h b/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.h
--- a/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.h
+++ b/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.h
@@ -36,6 +36,7 @@
   enum SIMDEnum {
     NoSIMD,
     SIMD128,
+    RelaxedSIMD,
   } SIMDLevel = NoSIMD;
 
   bool HasAtomics = false;
@@ -89,6 +90,7 @@
   // Predicates used by WebAssemblyInstrInfo.td.
   bool hasAddr64() const { return TargetTriple.isArch64Bit(); }
   bool hasSIMD128() const { return SIMDLevel >= SIMD128; }
+  bool hasRelaxedSIMD() const { return SIMDLevel >= RelaxedSIMD; }
   bool hasAtomics() const { return HasAtomics; }
   bool hasNontrappingFPToInt() const { return HasNontrappingFPToInt; }
   bool hasSignExt() const { return HasSignExt; }