Index: libc/config/darwin/arm/entrypoints.txt
===================================================================
--- libc/config/darwin/arm/entrypoints.txt
+++ libc/config/darwin/arm/entrypoints.txt
@@ -35,6 +35,7 @@
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
     libc.src.string.strncat
Index: libc/config/linux/aarch64/entrypoints.txt
===================================================================
--- libc/config/linux/aarch64/entrypoints.txt
+++ libc/config/linux/aarch64/entrypoints.txt
@@ -40,6 +40,7 @@
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
     libc.src.string.strncat
Index: libc/config/linux/x86_64/entrypoints.txt
===================================================================
--- libc/config/linux/x86_64/entrypoints.txt
+++ libc/config/linux/x86_64/entrypoints.txt
@@ -40,6 +40,7 @@
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
     libc.src.string.strncat
Index: libc/config/windows/entrypoints.txt
===================================================================
--- libc/config/windows/entrypoints.txt
+++ libc/config/windows/entrypoints.txt
@@ -35,6 +35,7 @@
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
     libc.src.string.strncat
Index: libc/spec/bsd_ext.td
===================================================================
--- libc/spec/bsd_ext.td
+++ libc/spec/bsd_ext.td
@@ -5,6 +5,11 @@
       [], // Types
       [], // Enumerations
       [ 
+        FunctionSpec<
+            "strlcat",
+            RetValSpec<SizeTType>,
+            [ArgSpec<CharPtr>, ArgSpec<ConstCharPtr>, ArgSpec<SizeTType>]
+        >,
         FunctionSpec<
             "strlcpy",
             RetValSpec<SizeTType>,
Index: libc/src/string/CMakeLists.txt
===================================================================
--- libc/src/string/CMakeLists.txt
+++ libc/src/string/CMakeLists.txt
@@ -6,6 +6,8 @@
     string_utils.h
   DEPENDS
     libc.src.__support.CPP.bitset
+    .memory_utils.memcpy_implementation
+    .memory_utils.memset_implementation
 )
 
 add_entrypoint_object(
@@ -126,6 +128,17 @@
     libc.include.stdlib
 )
 
+add_entrypoint_object(
+  strlcat
+  SRCS
+    strlcat.cpp
+  HDRS
+    strlcat.h
+  DEPENDS
+    .string_utils
+    libc.include.string
+)
+
 add_entrypoint_object(
   strlcpy
   SRCS
@@ -133,8 +146,6 @@
   HDRS
     strlcpy.h
   DEPENDS
-    .memory_utils.memcpy_implementation
-    .memory_utils.memset_implementation
     .string_utils
     libc.include.string
 )
Index: libc/src/string/string_utils.h
===================================================================
--- libc/src/string/string_utils.h
+++ libc/src/string/string_utils.h
@@ -11,6 +11,8 @@
 
 #include "src/__support/CPP/Bitset.h"
 #include "src/__support/common.h"
+#include "src/string/memory_utils/memcpy_implementations.h"
+#include "src/string/memory_utils/memset_implementations.h"
 #include <stddef.h> // size_t
 
 namespace __llvm_libc {
@@ -85,6 +87,17 @@
   return token;
 }
 
+static inline size_t strlcpy(char *__restrict dst, const char *__restrict src,
+                             size_t size) {
+  size_t len = internal::string_length(src);
+  if (!size)
+    return len;
+  size_t n = len < size - 1 ? len : size - 1;
+  inline_memcpy(dst, src, n);
+  inline_memset(dst + n, 0, size - n);
+  return len;
+}
+
 } // namespace internal
 } // namespace __llvm_libc
 
Index: libc/src/string/strlcat.h
===================================================================
--- /dev/null
+++ libc/src/string/strlcat.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for strlcat -----------------------*- 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_STRLCAT_H
+#define LLVM_LIBC_SRC_STRING_STRLCAT_H
+
+#include <string.h>
+
+namespace __llvm_libc {
+
+size_t strlcat(char *__restrict dst, const char *__restrict src, size_t size);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRLCAT_H
Index: libc/src/string/strlcat.cpp
===================================================================
--- /dev/null
+++ libc/src/string/strlcat.cpp
@@ -0,0 +1,27 @@
+//===-- Implementation of strlcat -----------------------------------------===//
+//
+// 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/strlcat.h"
+#include "src/string/string_utils.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(size_t, strlcat,
+                   (char *__restrict dst, const char *__restrict src,
+                    size_t size)) {
+  char *new_dst = reinterpret_cast<char *>(internal::find_first_character(
+      reinterpret_cast<unsigned char *>(dst), 0, size));
+  if (!new_dst)
+    return size + internal::string_length(src);
+  size_t first_len = new_dst - dst;
+  return first_len + internal::strlcpy(new_dst, src, size - first_len);
+}
+
+} // namespace __llvm_libc
Index: libc/src/string/strlcpy.cpp
===================================================================
--- libc/src/string/strlcpy.cpp
+++ libc/src/string/strlcpy.cpp
@@ -7,9 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/string/strlcpy.h"
-#include "src/string/bzero.h"
-#include "src/string/memory_utils/memcpy_implementations.h"
-#include "src/string/memory_utils/memset_implementations.h"
 #include "src/string/string_utils.h"
 
 #include "src/__support/common.h"
@@ -19,13 +16,7 @@
 LLVM_LIBC_FUNCTION(size_t, strlcpy,
                    (char *__restrict dst, const char *__restrict src,
                     size_t size)) {
-  size_t len = internal::string_length(src);
-  if (!size)
-    return len;
-  size_t n = len < size - 1 ? len : size - 1;
-  inline_memcpy(dst, src, n);
-  inline_memset(dst + n, 0, size - n);
-  return len;
+  return internal::strlcpy(dst, src, size);
 }
 
 } // namespace __llvm_libc
Index: libc/test/src/string/CMakeLists.txt
===================================================================
--- libc/test/src/string/CMakeLists.txt
+++ libc/test/src/string/CMakeLists.txt
@@ -123,6 +123,16 @@
     libc.src.string.strdup
 )
 
+add_libc_unittest(
+  strlcat_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strlcat_test.cpp
+  DEPENDS
+    libc.src.string.strlcat
+)
+
 add_libc_unittest(
   strlcpy_test
   SUITE
Index: libc/test/src/string/strlcat_test.cpp
===================================================================
--- /dev/null
+++ libc/test/src/string/strlcat_test.cpp
@@ -0,0 +1,37 @@
+//===-- Unittests for strlcat ---------------------------------------------===//
+//
+// 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/strlcat.h"
+#include "utils/UnitTest/Test.h"
+#include <stdlib.h>
+
+TEST(LlvmLibcStrlcatTest, TooBig) {
+  const char *str = "cd";
+  char buf[4]{"ab"};
+  EXPECT_EQ(__llvm_libc::strlcat(buf, str, 3), size_t(4));
+  EXPECT_STREQ(buf, "ab");
+  EXPECT_EQ(__llvm_libc::strlcat(buf, str, 4), size_t(4));
+  EXPECT_STREQ(buf, "abc");
+}
+
+TEST(LlvmLibcStrlcatTest, Smaller) {
+  const char *str = "cd";
+  char buf[7]{"ab"};
+
+  EXPECT_EQ(__llvm_libc::strlcat(buf, str, 7), size_t(4));
+  EXPECT_STREQ(buf, "abcd");
+}
+
+TEST(LlvmLibcStrlcatTest, No0) {
+  const char *str = "cd";
+  char buf[7]{"ab"};
+  EXPECT_EQ(__llvm_libc::strlcat(buf, str, 1), size_t(3));
+  EXPECT_STREQ(buf, "ab");
+  EXPECT_EQ(__llvm_libc::strlcat(buf, str, 2), size_t(4));
+  EXPECT_STREQ(buf, "ab");
+}