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
@@ -11,6 +11,7 @@
     libc.src.string.strlen
     libc.src.string.memchr
     libc.src.string.strchr
+    libc.str.string.strstr
 )
 
 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
@@ -29,6 +29,7 @@
     libc.src.string.strcmp
     libc.src.string.memchr
     libc.src.string.strchr
+    libc.src.string.strstr
 
     # 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
@@ -60,6 +60,16 @@
     strchr.h
 )
 
+add_entrypoint_object(
+  strstr
+  SRCS
+    strstr.cpp
+  HDRS
+    strstr.h
+  DEPENDS
+    .strlen
+)
+
 # 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/strstr.h b/libc/src/string/strstr.h
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strstr.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for strstr ------------------------*- 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_STRSTR_H
+#define LLVM_LIBC_SRC_STRING_STRSTR_H
+
+namespace __llvm_libc {
+
+char *strstr(const char *haystack, const char *needle);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRSTR_H
diff --git a/libc/src/string/strstr.cpp b/libc/src/string/strstr.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strstr.cpp
@@ -0,0 +1,35 @@
+//===-- Implementation of strstr ------------------------------------------===//
+//
+// 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/strstr.h"
+
+#include "src/__support/common.h"
+#include "src/string/strlen.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+
+// TODO: This is a simple brute force implementation that uses strlen.
+// This can (and will) be improved upon using well known string matching
+// algorithms such as Boyer-Moore.
+char *LLVM_LIBC_ENTRYPOINT(strstr)(const char *haystack, const char *needle) {
+  const size_t hsize = strlen(haystack);
+  const size_t nsize = strlen(needle);
+  if (nsize > hsize)
+    return nullptr;
+  size_t j;
+  for (size_t i = 0; i <= hsize - nsize; ++i) {
+    for (j = 0; j < nsize && haystack[i + j] == needle[j]; ++j)
+      ;
+    if (j == nsize)
+      return const_cast<char *>(haystack + i);
+  }
+  return nullptr;
+}
+
+} // 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
@@ -62,6 +62,16 @@
     libc.src.string.strchr
 )
 
+add_libc_unittest(
+  strstr_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strstr_test.cpp
+  DEPENDS
+    libc.src.string.strstr
+)
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
diff --git a/libc/test/src/string/strstr_test.cpp b/libc/test/src/string/strstr_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/strstr_test.cpp
@@ -0,0 +1,134 @@
+//===-- Unittests for strstr ----------------------------------------------===//
+//
+// 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/strstr.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrStrTest, NeedleNotInHaystack) {
+  const char *haystack = "12345";
+  const char *needle = "a";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), nullptr);
+}
+
+TEST(StrStrTest, NeedleIsEmptyString) {
+  const char *haystack = "12345";
+  const char *needle = "";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), haystack);
+}
+
+TEST(StrStrTest, HaystackIsEmptyString) {
+  const char *haystack = "";
+  const char *needle = "12345";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), nullptr);
+}
+
+TEST(StrStrTest, HaystackAndNeedleAreEmptyStrings) {
+  const char *haystack = "";
+  const char *needle = "";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "");
+}
+
+TEST(StrStrTest, HaystackAndNeedleAreSingleCharacters) {
+  const char *haystack = "a";
+  // Same characer returns that character.
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, /*needle=*/"a"), "a");
+  // Different character returns nullptr.
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, /*needle=*/"b"), nullptr);
+}
+
+TEST(StrStrTest, NeedleEqualToHaystack) {
+  const char *haystack = "12345";
+  const char *needle = "12345";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "12345");
+}
+
+TEST(StrStrTest, NeedleSmallerThanHaystack) {
+  const char *haystack = "12345";
+  const char *needle = "345";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "345");
+}
+
+TEST(StrStrTest, NeedleLargerThanHaystack) {
+  const char *haystack = "123";
+  const char *needle = "12345";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), nullptr);
+}
+
+TEST(StrStrTest, NeedleAtBeginning) {
+  const char *haystack = "12345";
+  const char *needle = "12";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "12345");
+}
+
+TEST(StrStrTest, NeedleInMiddle) {
+  const char *haystack = "abcdefghi";
+  const char *needle = "def";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "defghi");
+}
+
+TEST(StrStrTest, NeedleDirectlyBeforeNullTerminator) {
+  const char *haystack = "abcdefghi";
+  const char *needle = "ghi";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "ghi");
+}
+
+TEST(StrStrTest, NeedlePastNullTerminator) {
+  const char haystack[5] = {'1', '2', '\0', '3', '4'};
+  // Shouldn't find anything after the null terminator.
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, /*needle=*/"3"), nullptr);
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, /*needle=*/"4"), nullptr);
+}
+
+TEST(StrStrTest, PartialNeedle) {
+  const char *haystack = "la_ap_lap";
+  const char *needle = "lap";
+  // Shouldn't find la or ap.
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "lap");
+}
+
+TEST(StrStrTest, MisspelledNeedle) {
+  const char *haystack = "atalloftwocities...wait, tale";
+  const char *needle = "tale";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "tale");
+}
+
+TEST(StrStrTest, AnagramNeedle) {
+  const char *haystack = "dgo_ogd_god_odg_gdo_dog";
+  const char *needle = "dog";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, needle), "dog");
+}
+
+TEST(StrStrTest, MorphedNeedle) {
+  // Changes a single letter in the needle to mismatch with the haystack.
+  const char *haystack = "once upon a time";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, "time"), "time");
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, "lime"), nullptr);
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, "tome"), nullptr);
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, "tire"), nullptr);
+  ASSERT_STREQ(__llvm_libc::strstr(haystack, "timo"), nullptr);
+}
+
+TEST(StrStrTest, SummedDifferenceMethodUnsignedIntegers) {
+  // This is a bug found in a variation of the Rabin-Karp algorithm for
+  // sub-string search, which used unsigned integers instead of signed integers.
+  // When taking the difference between two characters, it is possible that it
+  // is negative. This bug resulted in returning nullptr when looking
+  // for "x", which should not be the case.
+  const char *haystack1 = "tx";
+  const char *needle1 = "x";
+  // First character of haystack: 't' = 116
+  // First character of needle:   'x' = 120
+  // 't' - 'x' = -4, which would lead to problems if using unsigned integers
+  // here.
+  ASSERT_STREQ(__llvm_libc::strstr(haystack1, needle1), "x");
+
+  // Similar case when switched.
+  const char *haystack2 = "xt";
+  const char *needle2 = "t";
+  ASSERT_STREQ(__llvm_libc::strstr(haystack2, needle2), "t");
+}