diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -59,6 +59,8 @@
 - Maximum _ExtInt size was decreased from 16,777,215 bits to 8,388,608 bits.
   Motivation for this was discussed in PR51829.
 
+- The builtin ``__builtin_bswap128`` was added.
+
 New Compiler Flags
 ------------------
 
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -512,6 +512,7 @@
 BUILTIN(__builtin_bswap16, "UsUs", "nc")
 BUILTIN(__builtin_bswap32, "UZiUZi", "nc")
 BUILTIN(__builtin_bswap64, "UWiUWi", "nc")
+BUILTIN(__builtin_bswap128, "ULLLiULLLi", "nc")
 
 BUILTIN(__builtin_bitreverse8, "UcUc", "nc")
 BUILTIN(__builtin_bitreverse16, "UsUs", "nc")
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11640,7 +11640,8 @@
 
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
-  case Builtin::BI__builtin_bswap64: {
+  case Builtin::BI__builtin_bswap64:
+  case Builtin::BI__builtin_bswap128: {
     APSInt Val;
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -2905,7 +2905,8 @@
   }
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
-  case Builtin::BI__builtin_bswap64: {
+  case Builtin::BI__builtin_bswap64:
+  case Builtin::BI__builtin_bswap128: {
     return RValue::get(emitUnaryBuiltin(*this, E, Intrinsic::bswap));
   }
   case Builtin::BI__builtin_bitreverse8:
diff --git a/clang/test/CodeGen/builtins.cpp b/clang/test/CodeGen/builtins.cpp
--- a/clang/test/CodeGen/builtins.cpp
+++ b/clang/test/CodeGen/builtins.cpp
@@ -20,6 +20,10 @@
 decltype(__builtin_bswap32(0)) bswap32 = 42;
 extern uint64_t bswap64;
 decltype(__builtin_bswap64(0)) bswap64 = 42;
+#ifdef __SIZEOF_INT128__
+extern __uint128_t bswap128;
+decltype(__builtin_bswap128(0)) bswap128 = 42;
+#endif
 
 #ifdef __clang__
 extern uint8_t bitrev8;
diff --git a/clang/test/Sema/constant-builtins-2.c b/clang/test/Sema/constant-builtins-2.c
--- a/clang/test/Sema/constant-builtins-2.c
+++ b/clang/test/Sema/constant-builtins-2.c
@@ -216,6 +216,7 @@
 int h3 = __builtin_bswap16(0x1234) == 0x3412 ? 1 : f();
 int h4 = __builtin_bswap32(0x1234) == 0x34120000 ? 1 : f();
 int h5 = __builtin_bswap64(0x1234) == 0x3412000000000000 ? 1 : f();
+int h6 = __builtin_bswap128(0x1234) == (((__int128)0x3412) << 112) ? 1 : f();
 extern long int bi0;
 extern __typeof__(__builtin_expect(0, 0)) bi0;