Wonder Boy III: Monster Lair (モンスターレア) is a Sega System 16 game in the Wonder Boy series. It is not to be confused with Wonder Boy III: The Dragon's Trap, which is actually the fourth game in the series. It was later ported to the Sega Mega Drive, and was brought to the TurboGrafx-CD as well, with the North American release being called just Monster Lair and lacking all Wonder Boy branding. The Mega Drive version was not released in North America.
Monster Lair is unlike other games in the Wonder Boy series, though it at least resembles the original more than it does the various Monster World games. Though it still contains platforming segments, Wonder Boy's sword fires projectiles, and the screen auto-scrolls to the right.