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
@@ -1,3 +1,11 @@
+add_libc_fuzzer(
+  strcmp_fuzz
+  SRCS
+    strcmp_fuzz.cpp
+  DEPENDS
+    libc.src.string.strcmp
+)
+
 add_libc_fuzzer(
   strcpy_fuzz
   SRCS
@@ -9,9 +17,10 @@
 )
 
 add_libc_fuzzer(
-  strcmp_fuzz
+  strstr_fuzz
   SRCS
-    strcmp_fuzz.cpp
+    strstr_fuzz.cpp
   DEPENDS
-    libc.src.string.strcmp
+    libc.src.string.strstr
+    libc.src.string.strlen
 )
diff --git a/libc/fuzzing/string/strstr_fuzz.cpp b/libc/fuzzing/string/strstr_fuzz.cpp
new file mode 100644
--- /dev/null
+++ b/libc/fuzzing/string/strstr_fuzz.cpp
@@ -0,0 +1,98 @@
+//===-- strstr_fuzz.cpp ---------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// Fuzzing test for llvm-libc strstr implementation.
+///
+//===----------------------------------------------------------------------===//
+
+#include "src/string/strcmp.h"
+#include "src/string/strlen.h"
+#include "src/string/strstr.h"
+#include <stddef.h>
+#include <stdint.h>
+
+// The general structure is to take the value of the first byte, set size1 to
+// that value, and add the null terminator. size2 will then contain the rest of
+// the bytes in data.
+// For example, with inputs (data={2, 6, 4, 8, 0}, size=5):
+//         size1: data[0] = 2
+//         data1: {2, 6} + '\0' = {2, 6, '\0'}
+//         size2: size - size1 = 3
+//         data2: {4, 8, '\0'}
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  // Verify the size is at least 1 and the data is null terminated.
+  if (!size || data[size - 1] != '\0')
+    return 0;
+
+  const size_t size1 = (data[0] <= size ? data[0] : size);
+  const size_t size2 = size - size1;
+
+  // The first size will always be at least 1 since
+  // we need to append the null terminator. The second size
+  // needs to be checked since it must also contain the null
+  // terminator.
+  if (!size2)
+    return 0;
+
+  // Copy the data into new containers.
+  // Add one to data1 for null terminator.
+  uint8_t *data1 = new uint8_t[size1 + 1];
+  uint8_t *data2 = new uint8_t[size2];
+  if (!data1 || !data2)
+    __builtin_trap();
+
+  size_t i;
+  for (i = 0; i < size1; ++i)
+    data1[i] = data[i];
+  data1[size1] = '\0'; // Add null terminator to data1.
+
+  for (size_t j = 0; j < size2; ++j)
+    data2[j] = data[i++];
+
+  const char *needle = reinterpret_cast<const char *>(data1);
+  const char *haystack = reinterpret_cast<const char *>(data2);
+  const char *actual_result = __llvm_libc::strstr(haystack, needle);
+
+  // A null terminator may exist earlier.
+  const size_t haystack_size = __llvm_libc::strlen(haystack);
+  const size_t needle_size = __llvm_libc::strlen(needle);
+
+  if (actual_result) {
+    // Verify that the first N characters of the result match the needle,
+    // where N is the length of needle.
+    for (size_t i = 0; i < needle_size; ++i) {
+      if (actual_result[i] != needle[i])
+        __builtin_trap();
+    }
+    const char *haystack_ptr = haystack;
+    // Verify that the result is the first occurence of the needle.
+    for (; haystack_ptr != actual_result; ++haystack_ptr) {
+      size_t equal_letter_count = 0;
+      for (size_t j = 0; j < needle_size && haystack_ptr[j]; ++j) {
+        equal_letter_count += (haystack_ptr[j] == needle[j]);
+      }
+      if (needle_size > 0 && equal_letter_count == needle_size)
+        __builtin_trap(); // There was an earlier occurence of the needle.
+    }
+  } else {
+    // No result was found. Verify that the needle doesn't exist within the
+    // haystack.
+    for (size_t i = 0; i + needle_size < haystack_size; ++i) {
+      size_t equal_letter_count = 0;
+      for (size_t j = 0; j < needle_size; ++j) {
+        equal_letter_count += (haystack[i + j] == needle[j]);
+      }
+      if (needle_size > 0 && equal_letter_count == needle_size)
+        __builtin_trap(); // There was an earlier occurence of the needle.
+    }
+  }
+
+  delete[] data1;
+  delete[] data2;
+  return 0;
+}