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
@@ -20,7 +20,9 @@
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "clang/Tooling/Execution.h"
 #include "clang/Tooling/Tooling.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/Signals.h"
 
 namespace clang {
@@ -35,9 +37,36 @@
                                        "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);
+
+bool filterURI(llvm::StringRef URI, llvm::StringRef IndexRoot) {
+  auto Parsed = URI::parse(URI);
+  return !Parsed || !Parsed->body().startswith(IndexRoot);
+}
+
+/// Returns true if \p Sym should be excluded from index.
+bool filterSymbol(const Symbol &Sym, llvm::StringRef IndexRoot) {
+  if (*Sym.CanonicalDeclaration.FileURI)
+    if (filterURI(Sym.CanonicalDeclaration.FileURI, IndexRoot))
+      return true;
+  if (*Sym.Definition.FileURI)
+    if (filterURI(Sym.Definition.FileURI, IndexRoot))
+      return true;
+  for (const auto &Header : Sym.IncludeHeaders)
+    if (!isLiteralInclude(Header.IncludeHeader))
+      if (filterURI(Header.IncludeHeader, IndexRoot))
+        return true;
+  return false;
+}
+
 class IndexActionFactory : public tooling::FrontendActionFactory {
 public:
-  IndexActionFactory(IndexFileIn &Result) : Result(Result) {}
+  IndexActionFactory(IndexFileIn &Result, llvm::StringRef IndexRoot)
+      : Result(Result), IndexRoot(IndexRoot) {}
 
   std::unique_ptr<FrontendAction> create() override {
     SymbolCollector::Options Opts;
@@ -56,10 +85,10 @@
         },
         [&](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) {
@@ -77,6 +106,38 @@
     Result.Symbols = std::move(Symbols).build();
     Result.Refs = std::move(Refs).build();
     Result.Relations = std::move(Relations).build();
+
+    // Post-filtering.
+    if (!IndexRoot.empty()) {
+      llvm::DenseSet<SymbolID> ExcludeIDs;
+      for (const auto &Sym : *Result.Symbols)
+        if (filterSymbol(Sym, IndexRoot))
+          ExcludeIDs.insert(Sym.ID);
+
+      llvm::errs() << "Total symbols collected: " << Result.Symbols->size()
+                   << ". Filtering out " << ExcludeIDs.size() << " of them.\n";
+
+      if (!ExcludeIDs.empty()) {
+        SymbolSlab::Builder FilteredSymbols;
+        RefSlab::Builder FilteredRefs;
+        RelationSlab::Builder FilteredRelations;
+        for (const auto &Sym : *Result.Symbols)
+          if (ExcludeIDs.find(Sym.ID) == ExcludeIDs.end())
+            FilteredSymbols.insert(Sym);
+        for (const auto &SymbolAndRefs : *Result.Refs)
+          if (ExcludeIDs.find(SymbolAndRefs.first) == ExcludeIDs.end())
+            for (const auto &Ref : SymbolAndRefs.second)
+              FilteredRefs.insert(SymbolAndRefs.first, Ref);
+        for (const auto &Rel : *Result.Relations)
+          if ((ExcludeIDs.find(Rel.Object) == ExcludeIDs.end()) &&
+              (ExcludeIDs.find(Rel.Subject) == ExcludeIDs.end()))
+            FilteredRelations.insert(Rel);
+
+        Result.Symbols = std::move(FilteredSymbols).build();
+        Result.Refs = std::move(FilteredRefs).build();
+        Result.Relations = std::move(FilteredRelations).build();
+      }
+    }
   }
 
 private:
@@ -85,6 +146,7 @@
   SymbolSlab::Builder Symbols;
   RefSlab::Builder Refs;
   RelationSlab::Builder Relations;
+  std::string IndexRoot;
 };
 
 } // namespace
@@ -119,7 +181,9 @@
   // 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) {
     llvm::errs() << llvm::toString(std::move(Err)) << "\n";