Index: clang/lib/Sema/AnalysisBasedWarnings.cpp
===================================================================
--- clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -1140,6 +1140,10 @@
           ++AnnotatedCnt;
           continue; // Fallthrough annotation, good.
         }
+        if (isFollowedByFallThroughComment(LastStmt)) {
+          ++AnnotatedCnt;
+          continue; // Fallthrough comment, good.
+        }
 
         if (!LastStmt) { // This block contains no executable statements.
           // Traverse its predecessors.
@@ -1208,6 +1212,27 @@
       return nullptr;
     }
 
+    bool isFollowedByFallThroughComment(const Stmt *stmt) const {
+      // Crude detection of whether the fallthough is marked by a comment like
+      // /*FALLTHOUGH*/
+      bool Invalid;
+      const char *sourceData =
+          S.getSourceManager().getCharacterData(stmt->getEndLoc(), &Invalid);
+      if (Invalid)
+        return false;
+      const char *lineStart = strchr(sourceData, '\n');
+      if (lineStart == nullptr)
+        return false;
+      ++lineStart; // start of next line
+      const char *lineEnd = strchr(lineStart, '\n');
+      StringRef line(lineStart,
+                     lineEnd ? lineEnd - lineStart : strlen(lineStart));
+      llvm::Regex regex("(/\\*[ \\t]*fall[ -]?thr(ough|u)[ \\t]*\\*/)"
+                        "|(//[ \\t]*fall[ -]?thr(ough|u)[ \\t]*)",
+                        llvm::Regex::IgnoreCase);
+      return regex.match(line);
+    }
+
     bool FoundSwitchStatements;
     AttrStmts FallthroughStmts;
     Sema &S;
Index: clang/test/Sema/fallthrough-comment.c
===================================================================
--- /dev/null
+++ clang/test/Sema/fallthrough-comment.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wimplicit-fallthrough %s
+
+int fallthrough_comment(int n) {
+  switch (n) {
+  case 0:
+    n++;
+    // FALLTHROUGH
+  case 1:
+    n++;
+    /*fall-through*/
+  case 2:
+    n++;
+  case 3: // expected-warning{{unannotated fall-through between switch labels}} expected-note{{insert '__attribute__((fallthrough));' to silence this warning}} expected-note{{insert 'break;' to avoid fall-through}}
+    n++;
+    break;
+  }
+  return n;
+}