diff --git a/libc/config/baremetal/arm/entrypoints.txt b/libc/config/baremetal/arm/entrypoints.txt
--- a/libc/config/baremetal/arm/entrypoints.txt
+++ b/libc/config/baremetal/arm/entrypoints.txt
@@ -38,6 +38,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/darwin/arm/entrypoints.txt b/libc/config/darwin/arm/entrypoints.txt
--- a/libc/config/darwin/arm/entrypoints.txt
+++ b/libc/config/darwin/arm/entrypoints.txt
@@ -38,6 +38,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/darwin/x86_64/entrypoints.txt b/libc/config/darwin/x86_64/entrypoints.txt
--- a/libc/config/darwin/x86_64/entrypoints.txt
+++ b/libc/config/darwin/x86_64/entrypoints.txt
@@ -32,6 +32,7 @@
     libc.src.string.stpncpy
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/gpu/entrypoints.txt b/libc/config/gpu/entrypoints.txt
--- a/libc/config/gpu/entrypoints.txt
+++ b/libc/config/gpu/entrypoints.txt
@@ -34,6 +34,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -47,6 +47,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/linux/arm/entrypoints.txt b/libc/config/linux/arm/entrypoints.txt
--- a/libc/config/linux/arm/entrypoints.txt
+++ b/libc/config/linux/arm/entrypoints.txt
@@ -38,6 +38,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/config/linux/riscv64/entrypoints.txt b/libc/config/linux/riscv64/entrypoints.txt
--- a/libc/config/linux/riscv64/entrypoints.txt
+++ b/libc/config/linux/riscv64/entrypoints.txt
@@ -47,6 +47,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcoll
     libc.src.string.strcpy
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -47,6 +47,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcoll
     libc.src.string.strcpy
diff --git a/libc/config/windows/entrypoints.txt b/libc/config/windows/entrypoints.txt
--- a/libc/config/windows/entrypoints.txt
+++ b/libc/config/windows/entrypoints.txt
@@ -35,6 +35,7 @@
     libc.src.string.strcasestr
     libc.src.string.strcat
     libc.src.string.strchr
+    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
diff --git a/libc/spec/gnu_ext.td b/libc/spec/gnu_ext.td
--- a/libc/spec/gnu_ext.td
+++ b/libc/spec/gnu_ext.td
@@ -73,6 +73,11 @@
             RetValSpec<CharPtr>,
             [ArgSpec<ConstCharPtr>, ArgSpec<ConstCharPtr>]
         >,
+        FunctionSpec<
+            "strchrnul",
+            RetValSpec<CharPtr>,
+            [ArgSpec<ConstCharPtr>, ArgSpec<ConstCharPtr>]
+        >,
       ]
   >;
 
diff --git a/libc/src/string/CMakeLists.txt b/libc/src/string/CMakeLists.txt
--- a/libc/src/string/CMakeLists.txt
+++ b/libc/src/string/CMakeLists.txt
@@ -107,6 +107,17 @@
     strchr.h
 )
 
+add_entrypoint_object(
+  strchrnul
+  SRCS
+    strchrnul.cpp
+  HDRS
+    strchrnul.h
+  DEPENDS
+    .strchr
+    .string_utils
+)
+
 add_entrypoint_object(
   strcmp
   SRCS
diff --git a/libc/src/string/strchrnul.h b/libc/src/string/strchrnul.h
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strchrnul.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for strchrnul --------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_STRCHRNUL_H
+#define LLVM_LIBC_SRC_STRING_STRCHRNUL_H
+
+namespace __llvm_libc {
+
+char *strchrnul(const char *src, int c);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRCHRNUL_H
diff --git a/libc/src/string/strchrnul.cpp b/libc/src/string/strchrnul.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strchrnul.cpp
@@ -0,0 +1,22 @@
+//===-- Implementation of strchrnul --------------------------------------===//
+//
+// 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 "src/string/strchrnul.h"
+#include "src/string/strchr.h"
+#include "src/string/string_utils.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(char *, strchrnul, (const char *src, int c)) {
+  char *ch = __llvm_libc::strchr(src, c);
+  return ch ? ch : const_cast<char *>(src) + internal::string_length(src);
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -94,6 +94,16 @@
     libc.src.string.strchr
 )
 
+add_libc_unittest(
+  strchrnul_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strchrnul_test.cpp
+  DEPENDS
+    libc.src.string.strchrnul
+)
+
 add_libc_unittest(
   strcmp_test
   SUITE
diff --git a/libc/test/src/string/strchr_test.cpp b/libc/test/src/string/strchr_test.cpp
--- a/libc/test/src/string/strchr_test.cpp
+++ b/libc/test/src/string/strchr_test.cpp
@@ -58,7 +58,7 @@
   // Same case for when the character is not found.
   __llvm_libc::strchr(src, 'z');
   ASSERT_STREQ(src, "abcde");
-  // Same case for when looking for nullptr.
+  // Same case for when looking for null terminator.
   __llvm_libc::strchr(src, '\0');
   ASSERT_STREQ(src, "abcde");
 }
diff --git a/libc/test/src/string/strchrnul_test.cpp b/libc/test/src/string/strchrnul_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/strchrnul_test.cpp
@@ -0,0 +1,96 @@
+//===-- Unittests for strchrnul -------------------------------------------===//
+//
+// 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 "src/string/strchrnul.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcStrChrNulTest, FindsFirstCharacter) {
+  const char *src = "abcde";
+
+  // Should return original string since 'a' is the first character.
+  ASSERT_STREQ(__llvm_libc::strchrnul(src, 'a'), "abcde");
+  // Source string should not change.
+  ASSERT_STREQ(src, "abcde");
+}
+
+TEST(LlvmLibcStrChrNulTest, FindsMiddleCharacter) {
+  const char *src = "abcde";
+
+  // Should return characters after (and including) 'c'.
+  ASSERT_STREQ(__llvm_libc::strchrnul(src, 'c'), "cde");
+  // Source string should not change.
+  ASSERT_STREQ(src, "abcde");
+}
+
+TEST(LlvmLibcStrChrNulTest, FindsLastCharacterThatIsNotNullTerminator) {
+  const char *src = "abcde";
+
+  // Should return 'e' and null-terminator.
+  ASSERT_STREQ(__llvm_libc::strchrnul(src, 'e'), "e");
+  // Source string should not change.
+  ASSERT_STREQ(src, "abcde");
+}
+
+TEST(LlvmLibcStrChrNulTest, FindsNullTerminator) {
+  const char *src = "abcde";
+
+  // Should return null terminator.
+  ASSERT_STREQ(__llvm_libc::strchrnul(src, '\0'), "");
+  // Source string should not change.
+  ASSERT_STREQ(src, "abcde");
+}
+
+TEST(LlvmLibcStrChrNulTest,
+     CharacterNotWithinStringShouldReturnNullTerminator) {
+  const char *src = "123?";
+
+  // Since 'z' is not within the string, should return a pointer to the source
+  // string's null terminator.
+  char *result = __llvm_libc::strchrnul(src, 'z');
+  ASSERT_EQ(*result, '\0');
+
+  char *term = const_cast<char *>(src) + 4;
+  ASSERT_EQ(result, term);
+}
+
+TEST(LlvmLibcStrChrNulTest, TheSourceShouldNotChange) {
+  const char *src = "abcde";
+  // When the character is found, the source string should not change.
+  __llvm_libc::strchrnul(src, 'd');
+  ASSERT_STREQ(src, "abcde");
+  // Same case for when the character is not found.
+  __llvm_libc::strchrnul(src, 'z');
+  ASSERT_STREQ(src, "abcde");
+  // Same case for when looking for null terminator.
+  __llvm_libc::strchrnul(src, '\0');
+  ASSERT_STREQ(src, "abcde");
+}
+
+TEST(LlvmLibcStrChrNulTest, ShouldFindFirstOfDuplicates) {
+  // '1' is duplicated in the string, but it should find the first copy.
+  ASSERT_STREQ(__llvm_libc::strchrnul("abc1def1ghi", '1'), "1def1ghi");
+
+  const char *dups = "XXXXX";
+  // Should return original string since 'X' is the first character.
+  ASSERT_STREQ(__llvm_libc::strchrnul(dups, 'X'), dups);
+}
+
+TEST(LlvmLibcStrChrNulTest, EmptyStringShouldOnlyMatchNullTerminator) {
+  // Null terminator should match.
+  ASSERT_STREQ(__llvm_libc::strchrnul("", '\0'), "");
+
+  // All other characters should not match.
+  char *result = __llvm_libc::strchrnul("", 'Z');
+  ASSERT_EQ(*result, '\0');
+
+  result = __llvm_libc::strchrnul("", '3');
+  ASSERT_EQ(*result, '\0');
+
+  result = __llvm_libc::strchrnul("", '*');
+  ASSERT_EQ(*result, '\0');
+}