diff --git a/libc/shared/builtins.h b/libc/shared/builtins.h
index 338755314d309..0bb156131423d 100644
--- a/libc/shared/builtins.h
+++ b/libc/shared/builtins.h
@@ -18,5 +18,6 @@
 #include "libc_common.h"
 
 #include "builtins/addtf3.h"
+#include "builtins/subtf3.h"
 
 #endif // LLVM_LIBC_SHARED_BUILTINS_H
diff --git a/libc/shared/builtins/subtf3.h b/libc/shared/builtins/subtf3.h
new file mode 100644
index 0000000000000..a7cecef8b0479
--- /dev/null
+++ b/libc/shared/builtins/subtf3.h
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This header exposes LLVM-libc's __subtf3 implementation as shared::subtf3 so
+/// that it can be reused by compiler-rt's builtins.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_BUILTINS_SUBTF3_H
+#define LLVM_LIBC_SHARED_BUILTINS_SUBTF3_H
+
+#include "include/llvm-libc-types/float128.h"
+
+#ifdef LIBC_TYPES_HAS_FLOAT128
+
+#include "shared/libc_common.h"
+#include "src/__support/builtins/subtf3.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using builtins::subtf3;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LIBC_TYPES_HAS_FLOAT128
+
+#endif // LLVM_LIBC_SHARED_BUILTINS_SUBTF3_H
diff --git a/libc/src/__support/builtins/CMakeLists.txt b/libc/src/__support/builtins/CMakeLists.txt
index c4982887d0a7c..d3d1d65babce8 100644
--- a/libc/src/__support/builtins/CMakeLists.txt
+++ b/libc/src/__support/builtins/CMakeLists.txt
@@ -7,3 +7,13 @@ add_header_library(
     libc.src.__support.FPUtil.generic.add_sub
     libc.src.__support.macros.config
 )
+
+add_header_library(
+  subtf3
+  HDRS
+    subtf3.h
+  DEPENDS
+    libc.include.llvm-libc-types.float128
+    libc.src.__support.FPUtil.generic.add_sub
+    libc.src.__support.macros.config
+)
diff --git a/libc/src/__support/builtins/subtf3.h b/libc/src/__support/builtins/subtf3.h
new file mode 100644
index 0000000000000..bd758cce0f5f6
--- /dev/null
+++ b/libc/src/__support/builtins/subtf3.h
@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This header exposes LLVM-libc's __subtf3 implementation as builtins::subtf3
+/// so that it can be reused by compiler-rt's builtins.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_BUILTINS_SUBTF3_H
+#define LLVM_LIBC_SRC___SUPPORT_BUILTINS_SUBTF3_H
+
+#include "include/llvm-libc-types/float128.h"
+
+#ifdef LIBC_TYPES_HAS_FLOAT128
+
+#include "src/__support/FPUtil/generic/add_sub.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace builtins {
+
+// Subtraction at float128 precision; mirrors compiler-rt's __subtf3.
+LIBC_INLINE float128 subtf3(float128 x, float128 y) {
+  return fputil::generic::sub<float128>(x, y);
+}
+
+} // namespace builtins
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LIBC_TYPES_HAS_FLOAT128
+
+#endif // LLVM_LIBC_SRC___SUPPORT_BUILTINS_SUBTF3_H
diff --git a/libc/test/shared/CMakeLists.txt b/libc/test/shared/CMakeLists.txt
index 7b75011009e55..98f787c7453f6 100644
--- a/libc/test/shared/CMakeLists.txt
+++ b/libc/test/shared/CMakeLists.txt
@@ -827,6 +827,7 @@ add_fp_unittest(
     shared_builtins_test.cpp
   DEPENDS
     libc.src.__support.builtins.addtf3
+    libc.src.__support.builtins.subtf3
 )
 
 add_fp_unittest(
diff --git a/libc/test/shared/shared_builtins_test.cpp b/libc/test/shared/shared_builtins_test.cpp
index 67d40b349cef3..bf84d2c0866a4 100644
--- a/libc/test/shared/shared_builtins_test.cpp
+++ b/libc/test/shared/shared_builtins_test.cpp
@@ -21,9 +21,10 @@ TEST(LlvmLibcSharedBuiltinsTest, AllDouble) {
 #ifdef LIBC_TYPES_HAS_FLOAT128
 
 TEST(LlvmLibcSharedBuiltinsTest, AllFloat128) {
-  namespace shared = LIBC_NAMESPACE::shared;
-
-  EXPECT_FP_EQ(float128(3.0), shared::addtf3(float128(1.0), float128(2.0)));
+  EXPECT_FP_EQ(float128(3.0),
+               LIBC_NAMESPACE::shared::addtf3(float128(1.0), float128(2.0)));
+  EXPECT_FP_EQ(float128(2.0),
+               LIBC_NAMESPACE::shared::subtf3(float128(5.0), float128(3.0)));
 }
 
 #endif // LIBC_TYPES_HAS_FLOAT128