diff --git a/clang-tools-extra/clangd/index/CanonicalIncludes.cpp b/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
--- a/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
+++ b/clang-tools-extra/clangd/index/CanonicalIncludes.cpp
@@ -92,6 +92,8 @@
 #include "StdSymbolMap.inc"
         // There are two std::move()s, this is by far the most common.
         SYMBOL(move, std::, <utility>)
+        // There are multiple headers for size_t, pick one.
+        SYMBOL(size_t, std::, <cstddef>)
 #undef SYMBOL
     });
     StdSymbolMapping = Symbols;
@@ -99,6 +101,8 @@
     static const auto *CSymbols = new llvm::StringMap<llvm::StringRef>({
 #define SYMBOL(Name, NameSpace, Header) {#Name, #Header},
 #include "CSymbolMap.inc"
+        // There are multiple headers for size_t, pick one.
+        SYMBOL(size_t, None, <stddef.h>)
 #undef SYMBOL
     });
     StdSymbolMapping = CSymbols;