diff --git a/clang/lib/Format/UnwrappedLineParser.h b/clang/lib/Format/UnwrappedLineParser.h
--- a/clang/lib/Format/UnwrappedLineParser.h
+++ b/clang/lib/Format/UnwrappedLineParser.h
@@ -97,7 +97,7 @@
   void parsePPEndIf();
   void parsePPUnknown();
   void readTokenWithJavaScriptASI();
-  void parseStructuralElement();
+  void parseStructuralElement(bool IsTopLevel = false);
   bool tryToParseBracedList();
   bool parseBracedList(bool ContinueOnSemicolons = false, bool IsEnum = false,
                        tok::TokenKind ClosingBraceKind = tok::r_brace);
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
@@ -431,7 +431,7 @@
       }
       LLVM_FALLTHROUGH;
     default:
-      parseStructuralElement();
+      parseStructuralElement(/*IsTopLevel=*/true);
       break;
     }
   } while (!eof());
@@ -994,6 +994,33 @@
       Keywords.kw_import, tok::kw_export);
 }
 
+// This function checks whether a token starts the first parameter declaration
+// in a K&R C (aka C78) function definition, e.g.:
+//   int f(a, b)
+//   short a, b;
+//   {
+//      return a + b;
+//   }
+static bool isC78ParameterDecl(const FormatToken *Tok) {
+  if (!Tok)
+    return false;
+
+  if (!Tok->isOneOf(tok::kw_int, tok::kw_char, tok::kw_float, tok::kw_double,
+                    tok::kw_struct, tok::kw_union, tok::kw_long, tok::kw_short,
+                    tok::kw_unsigned, tok::kw_register, tok::identifier))
+    return false;
+
+  Tok = Tok->Previous;
+  if (!Tok || Tok->isNot(tok::r_paren))
+    return false;
+
+  Tok = Tok->Previous;
+  if (!Tok || Tok->isNot(tok::identifier))
+    return false;
+
+  return Tok->Previous && Tok->Previous->isOneOf(tok::l_paren, tok::comma);
+}
+
 // readTokenWithJavaScriptASI reads the next token and terminates the current
 // line if JavaScript Automatic Semicolon Insertion must
 // happen between the current token and the next token.
@@ -1041,7 +1068,7 @@
     return addUnwrappedLine();
 }
 
-void UnwrappedLineParser::parseStructuralElement() {
+void UnwrappedLineParser::parseStructuralElement(bool IsTopLevel) {
   assert(!FormatTok->is(tok::l_brace));
   if (Style.Language == FormatStyle::LK_TableGen &&
       FormatTok->is(tok::pp_include)) {
@@ -1343,6 +1370,18 @@
       return;
     case tok::l_paren:
       parseParens();
+      // Break the unwrapped line if a K&R C function definition has a parameter
+      // declaration.
+      if (!IsTopLevel || !Style.isCpp())
+        break;
+      if (!Previous || Previous->isNot(tok::identifier))
+        break;
+      if (Previous->Previous && Previous->Previous->is(tok::at))
+        break;
+      if (isC78ParameterDecl(FormatTok)) {
+        addUnwrappedLine();
+        return;
+      }
       break;
     case tok::kw_operator:
       nextToken();
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
@@ -8216,7 +8216,16 @@
                "f(i)\n"
                "{\n"
                "  return i + 1;\n"
-               "}\n",
+               "}",
+               Style);
+  verifyFormat("int f(a, b, c);\n" // No break here.
+               "int\n"             // Break here.
+               "f(a, b, c)\n"      // Break here.
+               "short a, b;\n"
+               "float c;\n"
+               "{\n"
+               "  return a + b < c;\n"
+               "}",
                Style);
 
   Style = getGNUStyle();
@@ -9423,7 +9432,7 @@
   verifyFormat("vector<A(uint64_t) * attr> v;", TypeMacros); // multiplication
 
   FormatStyle CustomQualifier = getLLVMStyle();
-  // Add indentifers that should not be parsed as a qualifier by default.
+  // Add identifiers that should not be parsed as a qualifier by default.
   CustomQualifier.AttributeMacros.push_back("__my_qualifier");
   CustomQualifier.AttributeMacros.push_back("_My_qualifier");
   CustomQualifier.AttributeMacros.push_back("my_other_qualifier");