diff --git a/clang-tools-extra/clangd/indexer/IndexerMain.cpp b/clang-tools-extra/clangd/indexer/IndexerMain.cpp
--- a/clang-tools-extra/clangd/indexer/IndexerMain.cpp
+++ b/clang-tools-extra/clangd/indexer/IndexerMain.cpp
@@ -17,11 +17,15 @@
 #include "index/Symbol.h"
 #include "index/SymbolCollector.h"
 #include "support/Logger.h"
+#include "clang/Basic/SourceManager.h"
 #include "clang/Tooling/ArgumentsAdjusters.h"
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "clang/Tooling/Execution.h"
 #include "clang/Tooling/Tooling.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/Signals.h"
 
 namespace clang {
@@ -36,12 +40,34 @@
                                        "binary RIFF format")),
            llvm::cl::init(IndexFileFormat::RIFF));
 
+static llvm::cl::opt<std::string>
+    ProjectRoot("project-root",
+                llvm::cl::desc("Filter out all symbols outside of this "
+                               "directory. This is useful for remote index."),
+                llvm::cl::Hidden);
+
 class IndexActionFactory : public tooling::FrontendActionFactory {
 public:
-  IndexActionFactory(IndexFileIn &Result) : Result(Result) {}
+  IndexActionFactory(IndexFileIn &Result, llvm::StringRef IndexRoot)
+      : Result(Result) {
+    if (!IndexRoot.empty()) {
+      // Returns true if file should be indexed.
+      FileFilter = [&](const SourceManager &SM, FileID ID) {
+        const FileEntry *Entry = SM.getFileEntryForID(ID);
+        if (!Entry)
+          return false;
+        llvm::SmallString<256> FilePath = Entry->tryGetRealPathName();
+        // When replacing prefix it checks whether Path starts with OldPrefix.
+        // NewPrefix doesn't matter in this case.
+        return llvm::sys::path::replace_path_prefix(
+            /*Path=*/FilePath, /*OldPrefix=*/IndexRoot, /*NewPrefix=*/"");
+      };
+    }
+  }
 
   std::unique_ptr<FrontendAction> create() override {
     SymbolCollector::Options Opts;
+    Opts.FileFilter = FileFilter;
     Opts.CountReferences = true;
     return createStaticIndexingAction(
         Opts,
@@ -57,10 +83,10 @@
         },
         [&](RefSlab S) {
           std::lock_guard<std::mutex> Lock(SymbolsMu);
-          for (const auto &Sym : S) {
+          for (const auto &SymbolAndRefs : S) {
             // Deduplication happens during insertion.
-            for (const auto &Ref : Sym.second)
-              Refs.insert(Sym.first, Ref);
+            for (const auto &Ref : SymbolAndRefs.second)
+              Refs.insert(SymbolAndRefs.first, Ref);
           }
         },
         [&](RelationSlab S) {
@@ -86,6 +112,7 @@
   SymbolSlab::Builder Symbols;
   RefSlab::Builder Refs;
   RelationSlab::Builder Relations;
+  std::function<bool(const SourceManager &, FileID)> FileFilter = nullptr;
 };
 
 } // namespace
@@ -120,7 +147,9 @@
   // Collect symbols found in each translation unit, merging as we go.
   clang::clangd::IndexFileIn Data;
   auto Err = Executor->get()->execute(
-      std::make_unique<clang::clangd::IndexActionFactory>(Data),
+      std::make_unique<clang::clangd::IndexActionFactory>(
+          Data, llvm::sys::path::convert_to_slash(
+                    clang::clangd::ProjectRoot, llvm::sys::path::Style::posix)),
       clang::tooling::getStripPluginsAdjuster());
   if (Err) {
     clang::clangd::elog("{0}", std::move(Err));