diff --git a/libc/test/src/math/smoke/CMakeLists.txt b/libc/test/src/math/smoke/CMakeLists.txt
@@ -674,6 +674,22 @@ add_fp_unittest(
   UNIT_TEST_ONLY
 )
 
+add_fp_unittest(
+  copysignf128_test
+  SUITE
+    libc-math-smoke-tests
+  SRCS
+    copysignf128_test.cpp
+  HDRS
+    CopySignTest.h
+  DEPENDS
+    libc.include.math
+    libc.src.math.copysignf128
+    libc.src.__support.FPUtil.fp_bits
+  # FIXME: Currently fails on the GPU build.
+  UNIT_TEST_ONLY
+)
+
 add_fp_unittest(
   frexp_test
   SUITE

diff --git a/libc/test/src/math/smoke/CopySignTest.h b/libc/test/src/math/smoke/CopySignTest.h
@@ -20,28 +20,29 @@ class CopySignTest : public LIBC_NAMESPACE::testing::Test {
   typedef T (*CopySignFunc)(T, T);
 
   void testSpecialNumbers(CopySignFunc func) {
-    EXPECT_FP_EQ(aNaN, func(aNaN, -1.0));
-    EXPECT_FP_EQ(aNaN, func(aNaN, 1.0));
+    EXPECT_FP_EQ(aNaN, func(aNaN, T(-1.0)));
+    EXPECT_FP_EQ(aNaN, func(aNaN, T(1.0)));
 
-    EXPECT_FP_EQ(neg_inf, func(inf, -1.0));
-    EXPECT_FP_EQ(inf, func(neg_inf, 1.0));
+    EXPECT_FP_EQ(neg_inf, func(inf, T(-1.0)));
+    EXPECT_FP_EQ(inf, func(neg_inf, T(1.0)));
 
-    EXPECT_FP_EQ(neg_zero, func(zero, -1.0));
-    EXPECT_FP_EQ(zero, func(neg_zero, 1.0));
+    EXPECT_FP_EQ(neg_zero, func(zero, T(-1.0)));
+    EXPECT_FP_EQ(zero, func(neg_zero, T(1.0)));
   }
 
   void testRange(CopySignFunc func) {
     constexpr UIntType COUNT = 100'000;
     constexpr UIntType STEP = UIntType(-1) / COUNT;
     for (UIntType i = 0, v = 0; i <= COUNT; ++i, v += STEP) {
-      T x = T(FPBits(v));
-      if (isnan(x) || isinf(x))
+      FPBits x_bits = FPBits(v);
+      T x = T(v);
+      if (x_bits.is_nan() || x_bits.is_inf())
         continue;
 
-      double res1 = func(x, -x);
+      T res1 = func(x, -x);
       ASSERT_FP_EQ(res1, -x);
 
-      double res2 = func(x, x);
+      T res2 = func(x, x);
       ASSERT_FP_EQ(res2, x);
     }
   }

diff --git a/libc/test/src/math/smoke/copysignf128_test.cpp b/libc/test/src/math/smoke/copysignf128_test.cpp
@@ -0,0 +1,13 @@
+//===-- Unittests for copysignf128 ----------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "CopySignTest.h"
+
+#include "src/math/copysignf128.h"
+
+LIST_COPYSIGN_TESTS(float128, LIBC_NAMESPACE::copysignf128)
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -308,6 +308,7 @@ libc_support_library(
     deps = [
         ":__support_macros_attributes",
         ":__support_macros_config",
+        ":__support_macros_properties_compiler",
     ],
 )
 
@@ -1821,6 +1822,8 @@ libc_math_function(name = "copysignf")
 
 libc_math_function(name = "copysignl")
 
+libc_math_function(name = "copysignf128")
+
 libc_math_function(name = "ilogb")
 
 libc_math_function(name = "ilogbf")