diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
--- a/clang-tools-extra/clangd/Diagnostics.cpp
+++ b/clang-tools-extra/clangd/Diagnostics.cpp
@@ -22,6 +22,8 @@
 #include "clang/Lex/Token.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
@@ -328,14 +330,22 @@
 void toLSPDiags(
     const Diag &D, const URIForFile &File, const ClangdDiagnosticOptions &Opts,
     llvm::function_ref<void(clangd::Diagnostic, llvm::ArrayRef<Fix>)> OutFn) {
-  auto FillBasicFields = [](const DiagBase &D) -> clangd::Diagnostic {
-    clangd::Diagnostic Res;
-    Res.range = D.Range;
-    Res.severity = getSeverity(D.Severity);
-    return Res;
-  };
+  clangd::Diagnostic Main;
+  Main.severity = getSeverity(D.Severity);
+
+  // Main diagnostic should always refer to a range inside main file. If a
+  // diagnostic made it so for, it means either itself or one of its notes is
+  // inside main file.
+  if (D.InsideMainFile) {
+    Main.range = D.Range;
+  } else {
+    auto It =
+        llvm::find_if(D.Notes, [](const Note &N) { return N.InsideMainFile; });
+    assert(It != D.Notes.end() &&
+           "neither the main diagnostic nor notes are inside main file");
+    Main.range = It->Range;
+  }
 
-  clangd::Diagnostic Main = FillBasicFields(D);
   Main.code = D.Name;
   switch (D.Source) {
   case Diag::Clang:
@@ -379,7 +389,9 @@
     for (auto &Note : D.Notes) {
       if (!Note.InsideMainFile)
         continue;
-      clangd::Diagnostic Res = FillBasicFields(Note);
+      clangd::Diagnostic Res;
+      Res.severity = getSeverity(Note.Severity);
+      Res.range = Note.Range;
       Res.message = noteMessage(D, Note, Opts);
       OutFn(std::move(Res), llvm::ArrayRef<Fix>());
     }
diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -1014,6 +1014,29 @@
   EXPECT_THAT(TU.build().getDiagnostics(), UnorderedElementsAre());
 }
 
+TEST(ToLSPDiag, RangeIsInMain) {
+  ClangdDiagnosticOptions Opts;
+  clangd::Diag D;
+  D.Range = {pos(1, 2), pos(3, 4)};
+  D.Notes.emplace_back();
+  Note &N = D.Notes.back();
+  N.Range = {pos(2, 3), pos(3, 4)};
+
+  D.InsideMainFile = true;
+  N.InsideMainFile = false;
+  toLSPDiags(D, {}, Opts,
+             [&](clangd::Diagnostic LSPDiag, ArrayRef<clangd::Fix>) {
+               EXPECT_EQ(LSPDiag.range, D.Range);
+             });
+
+  D.InsideMainFile = false;
+  N.InsideMainFile = true;
+  toLSPDiags(D, {}, Opts,
+             [&](clangd::Diagnostic LSPDiag, ArrayRef<clangd::Fix>) {
+               EXPECT_EQ(LSPDiag.range, N.Range);
+             });
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang