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
@@ -815,10 +815,13 @@
 
         assert(Lines[i].size() > IndentPrefix.size());
         const auto FirstNonSpace = Lines[i][IndentPrefix.size()];
-        const auto AllowsSpaceChange =
-            SpacesInPrefix != 0 ||
-            (!NoSpaceBeforeFirstCommentChar() ||
-             (FirstNonSpace == '}' && FirstLineSpaceChange != 0));
+        const bool IsFormatComment = LineTok && switchesFormatting(*LineTok);
+        const bool LineRequiresLeadingSpace =
+            !NoSpaceBeforeFirstCommentChar() ||
+            (FirstNonSpace == '}' && FirstLineSpaceChange != 0);
+        const bool AllowsSpaceChange =
+            !IsFormatComment &&
+            (SpacesInPrefix != 0 || LineRequiresLeadingSpace);
 
         if (PrefixSpaceChange[i] > 0 && AllowsSpaceChange) {
           Prefix[i] = IndentPrefix.str();
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
@@ -91,6 +91,11 @@
                "// line 2\n"
                "void f() {}\n");
 
+  EXPECT_EQ("// comment\n"
+            "// clang-format on\n",
+            format("//comment\n"
+                   "// clang-format on\n"));
+
   verifyFormat("void f() {\n"
                "  // Doesn't do anything\n"
                "}");