Wonder Boy in Monster World, known in Japan as Wonder Boy V: Monster World III, is a side-scrolling action role-playing game originally developed by Westone and published by Sega for the Mega Drive/Genesis in 1991. It is the fifth game in the "Wonder Boy" series and the third game in the "Monster World" sub-series, following "Wonder Boy in Monster Land" ("Super Wonder Boy: Monster World" on the Japanese Sega Mark III) and "" ("Monster World II: Dragon no Wana" on the Japanese Game Gear).