diff --git a/libc/fuzzing/string/CMakeLists.txt b/libc/fuzzing/string/CMakeLists.txt
--- a/libc/fuzzing/string/CMakeLists.txt
+++ b/libc/fuzzing/string/CMakeLists.txt
@@ -4,5 +4,6 @@
     strcpy_fuzz.cpp
   DEPENDS
     strcpy
+    strlen
     memcpy
 )
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
@@ -9,6 +9,7 @@
   DEPENDS
     strcpy
     string_h
+    strlen
 )
 
 add_entrypoint_object(
@@ -19,9 +20,20 @@
     strcpy.h
   DEPENDS
     string_h
+    strlen
     memcpy
 )
 
+add_entrypoint_object(
+  strlen
+  SRCS
+    strlen.cpp
+  HDRS
+    strlen.h
+  DEPENDS
+    string_h
+)
+
 # ------------------------------------------------------------------------------
 # memcpy
 # ------------------------------------------------------------------------------
diff --git a/libc/src/string/strcat.cpp b/libc/src/string/strcat.cpp
--- a/libc/src/string/strcat.cpp
+++ b/libc/src/string/strcat.cpp
@@ -7,16 +7,17 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/string/strcat.h"
+#include "src/string/strcpy.h"
+#include "src/string/strlen.h"
 
 #include "src/__support/common.h"
-#include "src/string/strcpy.h"
 
 namespace __llvm_libc {
 
 char *LLVM_LIBC_ENTRYPOINT(strcat)(char *dest, const char *src) {
   // We do not yet have an implementaion of strlen in so we will use strlen
   // from another libc.
-  __llvm_libc::strcpy(dest + ::strlen(dest), src);
+  __llvm_libc::strcpy(dest + __llvm_libc::strlen(dest), src);
   return dest;
 }
 
diff --git a/libc/src/string/strcpy.cpp b/libc/src/string/strcpy.cpp
--- a/libc/src/string/strcpy.cpp
+++ b/libc/src/string/strcpy.cpp
@@ -8,6 +8,7 @@
 
 #include "src/string/strcpy.h"
 #include "src/string/memcpy.h"
+#include "src/string/strlen.h"
 
 #include "src/__support/common.h"
 
@@ -15,7 +16,7 @@
 
 char *LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {
   return reinterpret_cast<char *>(
-      __llvm_libc::memcpy(dest, src, ::strlen(src) + 1));
+      __llvm_libc::memcpy(dest, src, __llvm_libc::strlen(src) + 1));
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/string/strcpy.cpp b/libc/src/string/strlen.h
copy from libc/src/string/strcpy.cpp
copy to libc/src/string/strlen.h
--- a/libc/src/string/strcpy.cpp
+++ b/libc/src/string/strlen.h
@@ -1,4 +1,4 @@
-//===-------------------- Implementation of strcpy -----------------------===//
+//===----------------- Implementation header for strlen -------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -6,16 +6,15 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/string/strcpy.h"
-#include "src/string/memcpy.h"
+#ifndef LLVM_LIBC_SRC_STRING_STRLEN_H
+#define LLVM_LIBC_SRC_STRING_STRLEN_H
 
-#include "src/__support/common.h"
+#include "include/string.h"
 
 namespace __llvm_libc {
 
-char *LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {
-  return reinterpret_cast<char *>(
-      __llvm_libc::memcpy(dest, src, ::strlen(src) + 1));
-}
+size_t strlen(const char *src);
 
 } // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRLEN_H
diff --git a/libc/src/string/strcpy.cpp b/libc/src/string/strlen.cpp
copy from libc/src/string/strcpy.cpp
copy to libc/src/string/strlen.cpp
--- a/libc/src/string/strcpy.cpp
+++ b/libc/src/string/strlen.cpp
@@ -1,4 +1,4 @@
-//===-------------------- Implementation of strcpy -----------------------===//
+//===-------------------- Implementation of strlen ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -6,16 +6,19 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/string/strcpy.h"
 #include "src/string/memcpy.h"
+#include "src/string/strcpy.h"
 
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-char *LLVM_LIBC_ENTRYPOINT(strcpy)(char *dest, const char *src) {
-  return reinterpret_cast<char *>(
-      __llvm_libc::memcpy(dest, src, ::strlen(src) + 1));
+size_t LLVM_LIBC_ENTRYPOINT(strlen)(const char *src) {
+  const char *end = src;
+  while (*end != '\0') {
+    end++;
+  }
+  return end - 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
@@ -11,6 +11,7 @@
   DEPENDS
     strcat
     strcpy
+    strlen
     memcpy
 )
 
@@ -22,9 +23,20 @@
     strcpy_test.cpp
   DEPENDS
     strcpy
+    strlen
     memcpy
 )
 
+add_libc_unittest(
+  strlen_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strlen_test.cpp
+  DEPENDS
+    strlen
+)
+
 # Tests all implementations of memcpy that can run on the host.
 get_property(memcpy_implementations GLOBAL PROPERTY memcpy_implementations)
 foreach(memcpy_config_name IN LISTS memcpy_implementations)
diff --git a/libc/test/src/string/strlen_test.cpp b/libc/test/src/string/strlen_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/strlen_test.cpp
@@ -0,0 +1,24 @@
+//===----------------------- Unittests for strlen -------------------------===//
+//
+// 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/strlen.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrLenTest, EmptyString) {
+  const char *empty = "";
+
+  size_t result = __llvm_libc::strlen(empty);
+  ASSERT_EQ((size_t)0, result);
+}
+
+TEST(StrLenTest, AnyString) {
+  const char *any = "Hello World!";
+
+  size_t result = __llvm_libc::strlen(any);
+  ASSERT_EQ((size_t)12, result);
+}