diff --git a/clang/lib/Format/IntegerLiteralSeparatorFixer.cpp b/clang/lib/Format/IntegerLiteralSeparatorFixer.cpp
--- a/clang/lib/Format/IntegerLiteralSeparatorFixer.cpp
+++ b/clang/lib/Format/IntegerLiteralSeparatorFixer.cpp
@@ -113,7 +113,11 @@
       continue;
     }
     if (Style.isCpp()) {
-      if (const auto Pos = Text.find_first_of("_i"); Pos != StringRef::npos) {
+      // Hex alpha digits a-f/A-F must be at the end of the string literal.
+      StringRef Suffixes = "_himnsuyd";
+      if (const auto Pos =
+              Text.find_first_of(IsBase16 ? Suffixes.drop_back() : Suffixes);
+          Pos != StringRef::npos) {
         Text = Text.substr(0, Pos);
         Length = Pos;
       }
diff --git a/clang/unittests/Format/IntegerLiteralSeparatorTest.cpp b/clang/unittests/Format/IntegerLiteralSeparatorTest.cpp
--- a/clang/unittests/Format/IntegerLiteralSeparatorTest.cpp
+++ b/clang/unittests/Format/IntegerLiteralSeparatorTest.cpp
@@ -60,6 +60,24 @@
                "hil = 0xABCil;",
                Style);
 
+  verifyFormat("bd = 0b1'0000d;\n"
+               "dh = 1'234h;\n"
+               "dmin = 1'234min;\n"
+               "dns = 1'234ns;\n"
+               "ds = 1'234s;\n"
+               "dus = 1'234us;\n"
+               "hy = 0xA'BCy;",
+               "bd = 0b10000d;\n"
+               "dh = 1234h;\n"
+               "dmin = 1234min;\n"
+               "dns = 1234ns;\n"
+               "ds = 1234s;\n"
+               "dus = 1234us;\n"
+               "hy = 0xABCy;",
+               Style);
+
+  verifyFormat("hd = 0xAB'Cd;", "hd = 0xABCd;", Style);
+
   verifyFormat("d = 5'678_km;\n"
                "h = 0xD'EF_u16;",
                "d = 5678_km;\n"