diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -25,6 +25,7 @@
 #include "clang/Serialization/ASTReader.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
 using namespace clang;
 
 static bool MacroBodyEndsInBackslash(StringRef MacroBody) {
@@ -914,6 +915,12 @@
   Builder.defineMacro("__LONG_WIDTH__", Twine(TI.getLongWidth()));
   Builder.defineMacro("__LLONG_WIDTH__", Twine(TI.getLongLongWidth()));
 
+  // The max size of a _BitInt is limited in SemaType.cpp to this value. If
+  // someday a target needs to pick a different value for this macro, be sure
+  // to update Sema::BuildBitIntType() as well.
+  Builder.defineMacro("__BITINT_MAXWIDTH__",
+                      Twine(llvm::IntegerType::MAX_INT_BITS));
+
   DefineTypeSize("__SCHAR_MAX__", TargetInfo::SignedChar, TI, Builder);
   DefineTypeSize("__SHRT_MAX__", TargetInfo::SignedShort, TI, Builder);
   DefineTypeSize("__INT_MAX__", TargetInfo::SignedInt, TI, Builder);
diff --git a/clang/lib/Headers/limits.h b/clang/lib/Headers/limits.h
--- a/clang/lib/Headers/limits.h
+++ b/clang/lib/Headers/limits.h
@@ -78,6 +78,8 @@
 #define LONG_WIDTH   __LONG_WIDTH__
 #define ULLONG_WIDTH __LLONG_WIDTH__
 #define LLONG_WIDTH  __LLONG_WIDTH__
+
+#define BITINT_MAXWIDTH __BITINT_MAXWIDTH__
 #endif
 
 #ifdef __CHAR_UNSIGNED__  /* -funsigned-char */
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
@@ -2267,6 +2267,10 @@
     return QualType();
   }
 
+  // The max size of a _BitInt is exposed to the user via the
+  // __BITINT_MAXWIDTH__ macro from InitPreprocessor.cpp. If someday a target
+  // needs to pick a different max value, be sure to update
+  // InitializePredefinedMacros() as well.
   if (NumBits > llvm::IntegerType::MAX_INT_BITS) {
     Diag(Loc, diag::err_bit_int_max_size)
         << IsUnsigned << llvm::IntegerType::MAX_INT_BITS;
diff --git a/clang/test/Headers/limits.cpp b/clang/test/Headers/limits.cpp
--- a/clang/test/Headers/limits.cpp
+++ b/clang/test/Headers/limits.cpp
@@ -74,8 +74,11 @@
 _Static_assert(ULLONG_WIDTH / CHAR_BIT == sizeof(unsigned long long));
 _Static_assert(LLONG_WIDTH == ULLONG_WIDTH);
 _Static_assert(LLONG_WIDTH / CHAR_BIT == sizeof(signed long long));
+
+_Static_assert(BITINT_MAXWIDTH >= ULLONG_WIDTH);
 #else
 /* None of these are defined. */
 int BOOL_WIDTH, CHAR_WIDTH, SCHAR_WIDTH, UCHAR_WIDTH, USHRT_WIDTH, SHRT_WIDTH,
-    UINT_WIDTH, INT_WIDTH, ULONG_WIDTH, LONG_WIDTH, ULLONG_WIDTH, LLONG_WIDTH;
+    UINT_WIDTH, INT_WIDTH, ULONG_WIDTH, LONG_WIDTH, ULLONG_WIDTH, LLONG_WIDTH,
+    BITINT_MAXWIDTH;
 #endif
diff --git a/clang/test/Preprocessor/init-aarch64.c b/clang/test/Preprocessor/init-aarch64.c
--- a/clang/test/Preprocessor/init-aarch64.c
+++ b/clang/test/Preprocessor/init-aarch64.c
@@ -40,6 +40,7 @@
 // AARCH64-NEXT: #define __ATOMIC_SEQ_CST 5
 // AARCH64:      #define __BIGGEST_ALIGNMENT__ 16
 // AARCH64_BE-NEXT: #define __BIG_ENDIAN__ 1
+// AARCH64-NEXT: #define __BITINT_MAXWIDTH__ 8388608
 // AARCH64-NEXT: #define __BOOL_WIDTH__ 8
 // AARCH64_BE-NEXT: #define __BYTE_ORDER__ __ORDER_BIG_ENDIAN__
 // AARCH64_LE-NEXT: #define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
diff --git a/clang/test/Preprocessor/init.c b/clang/test/Preprocessor/init.c
--- a/clang/test/Preprocessor/init.c
+++ b/clang/test/Preprocessor/init.c
@@ -1520,6 +1520,7 @@
 // WEBASSEMBLY-NEXT:#define __ATOMIC_RELEASE 3
 // WEBASSEMBLY-NEXT:#define __ATOMIC_SEQ_CST 5
 // WEBASSEMBLY-NEXT:#define __BIGGEST_ALIGNMENT__ 16
+// WEBASSEMBLY-NEXT:#define __BITINT_MAXWIDTH__ 8388608
 // WEBASSEMBLY-NEXT:#define __BOOL_WIDTH__ 8
 // WEBASSEMBLY-NEXT:#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
 // WEBASSEMBLY-NEXT:#define __CHAR16_TYPE__ unsigned short