diff --git a/clang/lib/Format/BreakableToken.cpp b/clang/lib/Format/BreakableToken.cpp
--- a/clang/lib/Format/BreakableToken.cpp
+++ b/clang/lib/Format/BreakableToken.cpp
@@ -102,6 +102,17 @@
 
   static const auto kNumberedListRegexp = llvm::Regex("^[1-9][0-9]?\\.");
   while (SpaceOffset != StringRef::npos) {
+    // In C++ with the -Werror=comment option, having multiline comments (
+    // two consecutive comment lines, the first ending in `\`) is an
+    // error. Avoid introducing multiline comments by not allowing a break right
+    // after '\'.
+    if (Style.isCpp()) {
+      StringRef::size_type LastNonBlank =
+          Text.find_last_not_of(Blanks, SpaceOffset);
+      if (LastNonBlank != StringRef::npos && Text[LastNonBlank] == '\\')
+        SpaceOffset = Text.find_last_of(Blanks, LastNonBlank);
+    }
+
     // Do not split before a number followed by a dot: this would be interpreted
     // as a numbered list, which would prevent re-flowing in subsequent passes.
     if (kNumberedListRegexp.match(Text.substr(SpaceOffset).ltrim(Blanks)))
diff --git a/clang/unittests/Format/FormatTestComments.cpp b/clang/unittests/Format/FormatTestComments.cpp
--- a/clang/unittests/Format/FormatTestComments.cpp
+++ b/clang/unittests/Format/FormatTestComments.cpp
@@ -745,6 +745,20 @@
                    getLLVMStyleWithColumns(49)));
 }
 
+TEST_F(FormatTestComments, DontIntroduceMultilineComments) {
+  // Avoid introducing a multiline comment by breaking after `\`.
+  for (int ColumnLimit = 15; ColumnLimit <= 17; ++ColumnLimit) {
+    EXPECT_EQ(
+        "// aaaaaaaaaa\n"
+        "// \\ bb",
+        format("// aaaaaaaaaa \\ bb", getLLVMStyleWithColumns(ColumnLimit)));
+    EXPECT_EQ(
+        "// aaaaaaaaa\n"
+        "// \\  bb",
+        format("// aaaaaaaaa \\  bb", getLLVMStyleWithColumns(ColumnLimit)));
+  }
+}
+
 TEST_F(FormatTestComments, DontSplitLineCommentsWithPragmas) {
   FormatStyle Pragmas = getLLVMStyleWithColumns(30);
   Pragmas.CommentPragmas = "^ IWYU pragma:";