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
@@ -9,6 +9,7 @@
     libc.src.string.strcpy
     libc.src.string.strcat
     libc.src.string.strlen
+    libc.src.string.memchr
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
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
@@ -27,6 +27,7 @@
     libc.src.string.strcat
     libc.src.string.strlen
     libc.src.string.strcmp
+    libc.src.string.memchr
 
     # sys/mman.h entrypoints
     libc.src.sys.mman.mmap
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
@@ -44,6 +44,15 @@
     libc.include.string
 )
 
+add_entrypoint_object(
+  memchr
+  SRCS
+    memchr.cpp
+  HDRS
+    memchr.h
+)
+
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,
diff --git a/libc/src/string/memchr.h b/libc/src/string/memchr.h
new file mode 100644
--- /dev/null
+++ b/libc/src/string/memchr.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for memchr ------------------------*- 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_MEMCHR_H
+#define LLVM_LIBC_SRC_STRING_MEMCHR_H
+
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+void *memchr(const void *src, int c, size_t n);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_MEMCHR_H
diff --git a/libc/src/string/memchr.cpp b/libc/src/string/memchr.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/string/memchr.cpp
@@ -0,0 +1,23 @@
+//===-- Implementation of memchr ------------------------------------------===//
+//
+// 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/memchr.h"
+#include "src/__support/common.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+// TODO: Look at performance benefits of comparing words.
+void *LLVM_LIBC_ENTRYPOINT(memchr)(const void *src, int c, size_t n) {
+  const unsigned char *str = reinterpret_cast<const unsigned char *>(src);
+  for (; n && *str != c; --n, ++str)
+    ;
+  return n ? const_cast<unsigned char *>(str) : nullptr;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/CMakeLists.txt b/libc/test/src/CMakeLists.txt
--- a/libc/test/src/CMakeLists.txt
+++ b/libc/test/src/CMakeLists.txt
@@ -91,4 +91,4 @@
 target_link_libraries(libc-integration-test
   PRIVATE
   ${library_files}
-)
+)
\ No newline at end of file
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
@@ -42,6 +42,16 @@
     libc.src.string.strcmp
 )
 
+add_libc_unittest(
+  memchr_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    memchr_test.cpp
+  DEPENDS
+    libc.src.string.memchr
+)
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
@@ -66,4 +76,3 @@
 add_libc_multi_impl_test(memcpy SRCS memcpy_test.cpp)
 add_libc_multi_impl_test(memset SRCS memset_test.cpp)
 add_libc_multi_impl_test(bzero SRCS bzero_test.cpp)
-
diff --git a/libc/test/src/string/memchr_test.cpp b/libc/test/src/string/memchr_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/memchr_test.cpp
@@ -0,0 +1,113 @@
+//===-- Unittests for memchr ----------------------------------------------===//
+//
+// 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/memchr.h"
+#include "utils/UnitTest/Test.h"
+#include <stddef.h>
+
+// A helper function that calls memchr and abstracts away the explicit cast for
+// readability purposes.
+const char *call_memchr(const void *src, int c, size_t size) {
+  return reinterpret_cast<const char *>(__llvm_libc::memchr(src, c, size));
+}
+
+TEST(MemChrTest, FindsCharacterAfterNullTerminator) {
+  // memchr should continue searching after a null terminator.
+  const size_t size = 5;
+  const unsigned char src[size] = {'a', '\0', 'b', 'c', '\0'};
+  // Should return 'b', 'c', '\0' even when after null terminator.
+  ASSERT_STREQ(call_memchr(src, 'b', size), "bc");
+}
+
+TEST(MemChrTest, FindsCharacterInNonNullTerminatedCollection) {
+  const size_t size = 3;
+  const unsigned char src[size] = {'a', 'b', 'c'};
+  // Should return 'b', 'c'.
+  const char *ret = call_memchr(src, 'b', size);
+  ASSERT_EQ(ret[0], 'b');
+  ASSERT_EQ(ret[1], 'c');
+}
+
+TEST(MemChrTest, FindsFirstCharacter) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return original array since 'a' is the first character.
+  ASSERT_STREQ(call_memchr(src, 'a', size), "abcde");
+}
+
+TEST(MemChrTest, FindsMiddleCharacter) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return characters after (and including) 'c'.
+  ASSERT_STREQ(call_memchr(src, 'c', size), "cde");
+}
+
+TEST(MemChrTest, FindsLastCharacterThatIsNotNullTerminator) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return 'e' and null-terminator.
+  ASSERT_STREQ(call_memchr(src, 'e', size), "e");
+}
+
+TEST(MemChrTest, FindsNullTerminator) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return null terminator.
+  ASSERT_STREQ(call_memchr(src, '\0', size), "");
+}
+
+TEST(MemChrTest, CharacterNotWithinStringShouldReturnNullptr) {
+  const size_t size = 4;
+  const unsigned char src[size] = {'1', '2', '3', '?'};
+  // Since 'z' is not within 'characters', should return nullptr.
+  ASSERT_STREQ(call_memchr(src, 'z', size), nullptr);
+}
+
+TEST(MemChrTest, CharacterNotWithinSizeShouldReturnNullptr) {
+  const unsigned char src[5] = {'1', '2', '3', '4', '\0'};
+  // Since '4' is not the first or second character, this should return nullptr.
+  const size_t size = 2;
+  ASSERT_STREQ(call_memchr(src, '4', size), nullptr);
+}
+
+TEST(MemChrTest, TheSourceShouldNotChange) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  const char *src_copy = reinterpret_cast<const char *>(src);
+  // When the character is found, the source string should not change.
+  __llvm_libc::memchr(src, 'd', size);
+  ASSERT_STREQ(reinterpret_cast<const char *>(src), src_copy);
+  // Same case for when the character is not found.
+  __llvm_libc::memchr(src, 'z', size);
+  ASSERT_STREQ(reinterpret_cast<const char *>(src), src_copy);
+}
+
+TEST(MemChrTest, ShouldFindFirstOfDuplicates) {
+  const size_t size = 12; // 11 characters + null terminator.
+  const char *dups = "abc1def1ghi";
+  // 1 is duplicated in 'dups', but it should find the first copy.
+  ASSERT_STREQ(call_memchr(dups, '1', size), "1def1ghi");
+}
+
+TEST(MemChrTest, EmptyStringShouldOnlyMatchNullTerminator) {
+  const size_t size = 1; // Null terminator.
+  const char *empty_string = "";
+  // Null terminator should match.
+  ASSERT_STREQ(call_memchr(empty_string, '\0', size), "");
+  // All other characters should not match.
+  ASSERT_STREQ(call_memchr(empty_string, 'A', size), nullptr);
+  ASSERT_STREQ(call_memchr(empty_string, '9', size), nullptr);
+  ASSERT_STREQ(call_memchr(empty_string, '?', size), nullptr);
+}
+
+TEST(MemChrTest, SingleRepeatedCharacterShouldReturnFirst) {
+  const char *dups = "XXXXX";
+  const size_t size = 6; // 5 characters + null terminator.
+  // Should return original string since X is first character.
+  ASSERT_STREQ(call_memchr(dups, 'X', size), dups);
+}