diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -3549,6 +3549,8 @@
 
 void UnwrappedLineParser::readToken(int LevelDifference) {
   SmallVector<FormatToken *, 1> Comments;
+  bool PreviousWasComment = false;
+  bool FirstNonCommentOnLine = false;
   do {
     FormatTok = Tokens->getNextToken();
     assert(FormatTok);
@@ -3565,8 +3567,23 @@
       FormatTok->MustBreakBefore = true;
     }
 
+    auto IsFirstOnLine = [](const FormatToken &Tok) {
+      return Tok.HasUnescapedNewline || Tok.IsFirst;
+    };
+
+    if (PreviousWasComment) {
+      // Consider preprocessor directives preceded by block comments as first on
+      // line.
+      FirstNonCommentOnLine =
+          FirstNonCommentOnLine || IsFirstOnLine(*FormatTok);
+    } else {
+      FirstNonCommentOnLine = IsFirstOnLine(*FormatTok);
+    }
+
+    PreviousWasComment = FormatTok->Tok.is(tok::comment);
+
     while (!Line->InPPDirective && FormatTok->Tok.is(tok::hash) &&
-           (FormatTok->HasUnescapedNewline || FormatTok->IsFirst)) {
+           FirstNonCommentOnLine) {
       distributeComments(Comments, FormatTok);
       Comments.clear();
       // If there is an unfinished unwrapped line, we flush the preprocessor
@@ -3585,6 +3602,14 @@
         Line->Level += PPBranchLevel;
       flushComments(isOnNewLine(*FormatTok));
       parsePPDirective();
+      PreviousWasComment = FormatTok->Tok.is(tok::comment);
+      if (PreviousWasComment) {
+        // Consider preprocessor directives preceded by block comments as first
+        // on line.
+        FirstNonCommentOnLine |= IsFirstOnLine(*FormatTok);
+      } else {
+        FirstNonCommentOnLine = IsFirstOnLine(*FormatTok);
+      }
     }
 
     if (!PPStack.empty() && (PPStack.back().Kind == PP_Unreachable) &&
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -1797,6 +1797,17 @@
 
 TEST_F(FormatTest, UnderstandsMacros) {
   verifyFormat("#define A (parentheses)");
+  verifyFormat("/* comment */ #define A (parentheses)");
+  verifyFormat("/* comment */ /* another comment */ #define A (parentheses)");
+  // Even the partial code should never be merged.
+  EXPECT_EQ("/* comment */ #define A (parentheses)\n"
+            "#",
+            format("/* comment */ #define A (parentheses)\n"
+                   "#"));
+  verifyFormat("/* comment */ #define A (parentheses)\n"
+               "#\n");
+  verifyFormat("/* comment */ #define A (parentheses)\n"
+               "#define B (parentheses)");
   verifyFormat("#define true ((int)1)");
   verifyFormat("#define and(x)");
   verifyFormat("#define if(x) x");