diff --git a/clang-tools-extra/clangd/CodeCompletionStrings.cpp b/clang-tools-extra/clangd/CodeCompletionStrings.cpp
--- a/clang-tools-extra/clangd/CodeCompletionStrings.cpp
+++ b/clang-tools-extra/clangd/CodeCompletionStrings.cpp
@@ -12,6 +12,7 @@
 #include "clang/AST/RawCommentList.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
+#include "llvm/Support/JSON.h"
 #include <limits>
 #include <utility>
 
@@ -86,7 +87,12 @@
   assert(!Ctx.getSourceManager().isLoadedSourceLocation(RC->getBeginLoc()));
   std::string Doc =
       RC->getFormattedText(Ctx.getSourceManager(), Ctx.getDiagnostics());
-  return looksLikeDocComment(Doc) ? Doc : "";
+  if (!looksLikeDocComment(Doc))
+    return "";
+  // Clang requires source to be UTF-8, but doesn't enforce this in comments.
+  if (!llvm::json::isUTF8(Doc))
+    Doc = llvm::json::fixUTF8(Doc);
+  return Doc;
 }
 
 void getSignature(const CodeCompletionString &CCS, std::string *Signature,
diff --git a/clang-tools-extra/clangd/unittests/CodeCompletionStringsTests.cpp b/clang-tools-extra/clangd/unittests/CodeCompletionStringsTests.cpp
--- a/clang-tools-extra/clangd/unittests/CodeCompletionStringsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CodeCompletionStringsTests.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "CodeCompletionStrings.h"
+#include "TestTU.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -56,6 +57,14 @@
             "Annotation: Ano\n\nIs this brief?");
 }
 
+TEST_F(CompletionStringTest, GetDeclCommentBadUTF8) {
+  // <ff> is not a valid byte here, should be replaced by encoded <U+FFFD>.
+  auto TU = TestTU::withCode("/*x\xffy*/ struct X;");
+  auto AST = TU.build();
+  EXPECT_EQ("x\xef\xbf\xbdy",
+            getDeclComment(AST.getASTContext(), findDecl(AST, "X")));
+}
+
 TEST_F(CompletionStringTest, MultipleAnnotations) {
   Builder.AddAnnotation("Ano1");
   Builder.AddAnnotation("Ano2");