diff --git a/toolchain/lexer/lex_helpers.h b/toolchain/lexer/lex_helpers.h
index e9b53a79e681..f45c5af27b20 100644
--- a/toolchain/lexer/lex_helpers.h
+++ b/toolchain/lexer/lex_helpers.h
@@ -5,8 +5,6 @@
 #ifndef TOOLCHAIN_LEXER_LEX_HELPERS_H_
 #define TOOLCHAIN_LEXER_LEX_HELPERS_H_
 
-#include <optional>
-
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 
 namespace Carbon {
diff --git a/toolchain/semantics/BUILD b/toolchain/semantics/BUILD
index acbdfbdfd61b..eede08fd1f32 100644
--- a/toolchain/semantics/BUILD
+++ b/toolchain/semantics/BUILD
@@ -22,5 +22,6 @@ cc_test(
         "//toolchain/parser:parse_tree",
         "//toolchain/source:source_buffer",
         "@com_google_googletest//:gtest_main",
+        "@llvm-project//llvm:Support",
     ],
 )
diff --git a/toolchain/semantics/semantics_test.cpp b/toolchain/semantics/semantics_test.cpp
index 4f700e2f9096..ecae3b4ad01d 100644
--- a/toolchain/semantics/semantics_test.cpp
+++ b/toolchain/semantics/semantics_test.cpp
@@ -7,8 +7,7 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
-#include <optional>
-
+#include "llvm/ADT/Optional.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/parser/parse_tree.h"
@@ -26,9 +25,9 @@ class ParseTreeTest : public ::testing::Test {
     return Semantics::Analyze(*parse_tree, consumer);
   }
 
-  std::optional<SourceBuffer> source_buffer;
-  std::optional<TokenizedBuffer> tokenized_buffer;
-  std::optional<ParseTree> parse_tree;
+  llvm::Optional<SourceBuffer> source_buffer;
+  llvm::Optional<TokenizedBuffer> tokenized_buffer;
+  llvm::Optional<ParseTree> parse_tree;
   DiagnosticConsumer& consumer = ConsoleDiagnosticConsumer();
 };
 
diff --git a/toolchain/source/source_buffer.cpp b/toolchain/source/source_buffer.cpp
index 238a9bdf0ad8..3babd50c6d48 100644
--- a/toolchain/source/source_buffer.cpp
+++ b/toolchain/source/source_buffer.cpp
@@ -12,11 +12,11 @@
 #include <cerrno>
 #include <cstdint>
 #include <limits>
-#include <optional>
 #include <system_error>
 #include <variant>
 
 #include "common/check.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/Error.h"