diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3021,6 +3021,7 @@
 def err_attribute_too_few_arguments : Error<
   "%0 attribute takes at least %1 argument%s1">;
 def err_attribute_invalid_vector_type : Error<"invalid vector element type %0">;
+def err_attribute_invalid_bitint_vector_type : Error<"invalid vector element type %0, must be byte-sized and power of 2 _BitInt">;
 def err_attribute_invalid_matrix_type : Error<"invalid matrix element type %0">;
 def err_attribute_bad_neon_vector_size : Error<
   "Neon vector size must be 64 or 128 bits">;
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -4012,7 +4012,11 @@
 /// the specified element type and size. VectorType must be a built-in type.
 QualType ASTContext::getVectorType(QualType vecType, unsigned NumElts,
                                    VectorType::VectorKind VecKind) const {
-  assert(vecType->isBuiltinType());
+  assert(vecType->isBuiltinType() ||
+         (vecType->isBitIntType() &&
+          // Only support _BitInt elements with byte-sized power of 2 NumBits.
+          llvm::isPowerOf2_32(vecType->getAs<BitIntType>()->getNumBits()) &&
+          vecType->getAs<BitIntType>()->getNumBits() >= 8));
 
   // Check if we've already instantiated a vector of this type.
   llvm::FoldingSetNodeID ID;
@@ -4080,9 +4084,13 @@
 
 /// getExtVectorType - Return the unique reference to an extended vector type of
 /// the specified element type and size. VectorType must be a built-in type.
-QualType
-ASTContext::getExtVectorType(QualType vecType, unsigned NumElts) const {
-  assert(vecType->isBuiltinType() || vecType->isDependentType());
+QualType ASTContext::getExtVectorType(QualType vecType,
+                                      unsigned NumElts) const {
+  assert(vecType->isBuiltinType() || vecType->isDependentType() ||
+         (vecType->isBitIntType() &&
+          // Only support _BitInt elements with byte-sized power of 2 NumBits.
+          llvm::isPowerOf2_32(vecType->getAs<BitIntType>()->getNumBits()) &&
+          vecType->getAs<BitIntType>()->getNumBits() >= 8));
 
   // Check if we've already instantiated a vector of this type.
   llvm::FoldingSetNodeID ID;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2637,11 +2637,20 @@
   // can't already be a vector.
   if ((!CurType->isDependentType() &&
        (!CurType->isBuiltinType() || CurType->isBooleanType() ||
-        (!CurType->isIntegerType() && !CurType->isRealFloatingType()))) ||
+        (!CurType->isIntegerType() && !CurType->isRealFloatingType())) &&
+       !CurType->isBitIntType()) ||
       CurType->isArrayType()) {
     Diag(AttrLoc, diag::err_attribute_invalid_vector_type) << CurType;
     return QualType();
   }
+  // Only support _BitInt elements with byte-sized power of 2 NumBits.
+  if (CurType->isBitIntType()) {
+    unsigned NumBits = CurType->getAs<BitIntType>()->getNumBits();
+    if (!llvm::isPowerOf2_32(NumBits) || NumBits < 8) {
+      Diag(AttrLoc, diag::err_attribute_invalid_bitint_vector_type) << CurType;
+      return QualType();
+    }
+  }
 
   if (SizeExpr->isTypeDependent() || SizeExpr->isValueDependent())
     return Context.getDependentVectorType(CurType, SizeExpr, AttrLoc,
@@ -2707,12 +2716,21 @@
   // We explictly allow bool elements in ext_vector_type for C/C++.
   bool IsNoBoolVecLang = getLangOpts().OpenCL || getLangOpts().OpenCLCPlusPlus;
   if ((!T->isDependentType() && !T->isIntegerType() &&
-       !T->isRealFloatingType()) || T->isBitIntType() ||
+       !T->isRealFloatingType()) ||
       (IsNoBoolVecLang && T->isBooleanType())) {
     Diag(AttrLoc, diag::err_attribute_invalid_vector_type) << T;
     return QualType();
   }
 
+  // Only support _BitInt elements with byte-sized power of 2 NumBits.
+  if (T->isBitIntType()) {
+    unsigned NumBits = T->getAs<BitIntType>()->getNumBits();
+    if (!llvm::isPowerOf2_32(NumBits) || NumBits < 8) {
+      Diag(AttrLoc, diag::err_attribute_invalid_bitint_vector_type) << T;
+      return QualType();
+    }
+  }
+
   if (!ArraySize->isTypeDependent() && !ArraySize->isValueDependent()) {
     Optional<llvm::APSInt> vecSize = ArraySize->getIntegerConstantExpr(Context);
     if (!vecSize) {
diff --git a/clang/test/Sema/builtin-classify-type.c b/clang/test/Sema/builtin-classify-type.c
--- a/clang/test/Sema/builtin-classify-type.c
+++ b/clang/test/Sema/builtin-classify-type.c
@@ -29,6 +29,23 @@
   __attribute__((vector_size(16))) int vec;
   typedef __attribute__((ext_vector_type(4))) int evec_t;
   evec_t evec;
+  typedef _BitInt(8) int8_t3 __attribute__((ext_vector_type(3)));
+  int8_t3 t3;
+  typedef _BitInt(16) int16_t3 __attribute__((ext_vector_type(4)));
+  int16_t3 t4;
+  typedef _BitInt(32) int32_t3 __attribute__((ext_vector_type(5)));
+  int32_t3 t5;
+  typedef _BitInt(64) int64_t3 __attribute__((ext_vector_type(6)));
+  int64_t3 t6;
+  typedef _BitInt(8) vint8_t3 __attribute__((vector_size(3)));
+  vint8_t3 vt3;
+  typedef _BitInt(16) vint16_t3 __attribute__((vector_size(4)));
+  vint16_t3 vt4;
+  typedef _BitInt(32) vint32_t3 __attribute__((vector_size(8)));
+  vint32_t3 vt5;
+  typedef _BitInt(64) vint64_t3 __attribute__((vector_size(16)));
+  vint64_t3 vt6;
+
   _Atomic int atomic_i;
   _Atomic double atomic_d;
   _Complex int complex_i;
diff --git a/clang/test/SemaCXX/ext-int.cpp b/clang/test/SemaCXX/ext-int.cpp
--- a/clang/test/SemaCXX/ext-int.cpp
+++ b/clang/test/SemaCXX/ext-int.cpp
@@ -84,10 +84,22 @@
 };
 
 // Reject vector types:
-// expected-error@+1{{invalid vector element type '_BitInt(32)'}}
-typedef _BitInt(32) __attribute__((vector_size(16))) VecTy;
-// expected-error@+1{{invalid vector element type '_BitInt(32)'}}
-typedef _BitInt(32) __attribute__((ext_vector_type(32))) OtherVecTy;
+// expected-error@+1{{invalid vector element type '_BitInt(2)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(2) __attribute__((vector_size(16))) VecTy;
+// expected-error@+1{{invalid vector element type '_BitInt(2)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(2) __attribute__((ext_vector_type(32))) OtherVecTy;
+// expected-error@+1{{invalid vector element type '_BitInt(4)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(4) __attribute__((vector_size(16))) VecTy2;
+// expected-error@+1{{invalid vector element type '_BitInt(4)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(4) __attribute__((ext_vector_type(32))) OtherVecTy2;
+// expected-error@+1{{invalid vector element type '_BitInt(5)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(5) __attribute__((vector_size(16))) VecTy3;
+// expected-error@+1{{invalid vector element type '_BitInt(5)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(5) __attribute__((ext_vector_type(32))) OtherVecTy3;
+// expected-error@+1{{invalid vector element type '_BitInt(37)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(37) __attribute__((vector_size(16))) VecTy4;
+// expected-error@+1{{invalid vector element type '_BitInt(37)', must be byte-sized and power of 2 _BitInt}}
+typedef _BitInt(37) __attribute__((ext_vector_type(32))) OtherVecTy4;
 
 // Allow _Complex:
 _Complex _BitInt(3) Cmplx;