diff --git a/libc/Makefile b/libc/Makefile
index da4cfe0c..0263eda7 100644
--- a/libc/Makefile
+++ b/libc/Makefile
@@ -179,6 +179,7 @@ string/memccpy.o \
 string/memchr.o \
 string/memcmp.o \
 string/memcpy.o \
+string/memmem.o \
 string/memmove.o \
 string/memrchr.o \
 string/memset.o \
@@ -394,6 +395,7 @@ ioleast/writeall.o \
 ioleast/writeleast.o \
 langinfo/nl_langinfo_l.o \
 langinfo/nl_langinfo.o \
+locale/getlocalename_l.o \
 locale/localeconv.o \
 locale/setlocale.o \
 memusage/memusage.o \
diff --git a/libc/include/string.h b/libc/include/string.h
index ca908eb2..cc4eef2a 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -121,6 +121,7 @@ size_t strxfrm_l(char* __restrict, const char* __restrict, size_t, locale_t);
 
 /* Functions from POSIX 2024. */
 #if __USE_SORTIX || 202405L <= __USE_POSIX
+void* memmem(const void*, size_t, const void*, size_t);
 size_t strlcat(char* __restrict, const char* __restrict, size_t);
 size_t strlcpy(char* __restrict, const char* __restrict, size_t);
 #endif
diff --git a/libc/string/memmem.c b/libc/string/memmem.c
new file mode 100644
index 00000000..b6b30053
--- /dev/null
+++ b/libc/string/memmem.c
@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2024 Jonas 'Sortie' Termansen.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * string/memmem.c
+ * Locate a subsequence.
+ */
+
+#include <string.h>
+
+// TODO: Remember the fast algorithm and implemented it here and in strstr.
+void* memmem(const void* haystack, size_t haystack_size,
+             const void* needle, size_t needle_size)
+{
+	if ( !needle_size )
+		return haystack;
+	if ( haystack_size < needle_size )
+		return NULL;
+	for ( size_t i = 0; i < haystack_size - needle_size; i++ )
+	{
+		for ( size_t n = 0; n < needle_size; n++ )
+		{
+			if ( ((char*) haystack)[i + n] != ((char*) needle)[n] )
+				break;
+			if ( n + 1 == needle_size )
+				return (char*) haystack + i;
+		}
+	}
+	return NULL;
+}