Index: clang/lib/Format/BreakableToken.cpp
===================================================================
--- clang/lib/Format/BreakableToken.cpp
+++ clang/lib/Format/BreakableToken.cpp
@@ -41,8 +41,8 @@
 
 static StringRef getLineCommentIndentPrefix(StringRef Comment,
                                             const FormatStyle &Style) {
-  static const char *const KnownCStylePrefixes[] = {"///<", "//!<", "///", "//",
-                                                    "//!"};
+  static const char *const KnownCStylePrefixes[] = {"///<", "//!<", "///",
+                                                    "//",   "//!",  "//:"};
   static const char *const KnownTextProtoPrefixes[] = {"//", "#", "##", "###",
                                                        "####"};
   ArrayRef<const char *> KnownPrefixes(KnownCStylePrefixes);
Index: clang/unittests/Format/FormatTestComments.cpp
===================================================================
--- clang/unittests/Format/FormatTestComments.cpp
+++ clang/unittests/Format/FormatTestComments.cpp
@@ -702,6 +702,12 @@
                    "  // long 1 2 3 4 5 6\n"
                    "}",
                    getLLVMStyleWithColumns(20)));
+
+  EXPECT_EQ("//: A comment that\n"
+            "//: doesn't fit on\n"
+            "//: one line",
+            format("//: A comment that doesn't fit on one line",
+                   getLLVMStyleWithColumns(20)));
 }
 
 TEST_F(FormatTestComments, PreservesHangingIndentInCxxComments) {