diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -110,6 +110,7 @@
     /// If true, ClangdServer automatically indexes files in the current project
     /// on background threads. The index is stored in the project root.
     bool BackgroundIndex = false;
+    llvm::ThreadPriority BackgroundIndexPriority = llvm::ThreadPriority::Low;
 
     /// If set, use this index to augment code completion results.
     SymbolIndex *StaticIndex = nullptr;
diff --git a/clang-tools-extra/clangd/index/Background.h b/clang-tools-extra/clangd/index/Background.h
--- a/clang-tools-extra/clangd/index/Background.h
+++ b/clang-tools-extra/clangd/index/Background.h
@@ -136,6 +136,8 @@
     // Arbitrary value to ensure some concurrency in tests.
     // In production an explicit value is specified.
     size_t ThreadPoolSize = 4;
+    // Thread priority when indexing files.
+    llvm::ThreadPriority IndexingPriority = llvm::ThreadPriority::Low;
     // Callback that provides notifications as indexing makes progress.
     std::function<void(BackgroundQueue::Stats)> OnProgress = nullptr;
     // Function called to obtain the Context to use while indexing the specified
@@ -194,6 +196,7 @@
   // configuration
   const ThreadsafeFS &TFS;
   const GlobalCompilationDatabase &CDB;
+  llvm::ThreadPriority IndexingPriority;
   std::function<Context(PathRef)> ContextProvider;
 
   llvm::Error index(tooling::CompileCommand);
diff --git a/clang-tools-extra/clangd/index/Background.cpp b/clang-tools-extra/clangd/index/Background.cpp
--- a/clang-tools-extra/clangd/index/Background.cpp
+++ b/clang-tools-extra/clangd/index/Background.cpp
@@ -92,6 +92,7 @@
     const ThreadsafeFS &TFS, const GlobalCompilationDatabase &CDB,
     BackgroundIndexStorage::Factory IndexStorageFactory, Options Opts)
     : SwapIndex(std::make_unique<MemIndex>()), TFS(TFS), CDB(CDB),
+      IndexingPriority(Opts.IndexingPriority),
       ContextProvider(std::move(Opts.ContextProvider)),
       IndexedSymbols(IndexContents::All),
       Rebuilder(this, &IndexedSymbols, Opts.ThreadPoolSize),
@@ -165,6 +166,7 @@
       elog("Indexing {0} failed: {1}", Path, std::move(Error));
   });
   T.QueuePri = IndexFile;
+  T.ThreadPri = IndexingPriority;
   T.Tag = std::move(Tag);
   T.Key = Key;
   return T;
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -169,6 +169,21 @@
     init(true),
 };
 
+opt<llvm::ThreadPriority> BackgroundIndexPriority{
+    "background-index-priority",
+    cat(Features),
+    desc("Thread priority for building the background index. "
+         "The effect of this flag is OS-specific."),
+    values(clEnumValN(llvm::ThreadPriority::Background, "background",
+                      "Minimum priority, runs on idle CPUs. "
+                      "May leave 'performance' cores unused."),
+           clEnumValN(llvm::ThreadPriority::Low, "low",
+                      "Reduced priority compared to interactive work."),
+           clEnumValN(llvm::ThreadPriority::Default, "normal",
+                      "Same priority as other clangd work.")),
+    init(llvm::ThreadPriority::Low),
+};
+
 opt<bool> EnableClangTidy{
     "clang-tidy",
     cat(Features),
@@ -876,6 +891,7 @@
   }
 #endif
   Opts.BackgroundIndex = EnableBackgroundIndex;
+  Opts.BackgroundIndexPriority = BackgroundIndexPriority;
   Opts.ReferencesLimit = ReferencesLimit;
   auto PAI = createProjectAwareIndex(loadExternalIndex, Sync);
   if (StaticIdx) {