Wonder Boy (ワンダーボーイ, Wandā Bōi) and the later Monster World (モンスターワールド, Monsutā Wārudo) make up a series of video games published by Sega and developed by Westone Bit Entertainment (formerly Escape).

The series itself consists of the main Wonder Boy series, and the Monster World sub-series. Games may be part of one, the other, or both. This has resulted in a sometimes confusing naming structure resulting in titles like "Wonder Boy V: Monster World III". North America and Europe received every game that was under the Wonder Boy brand, but Japan received one exclusive title that is exclusively part of Monster World.

To further complicate matters, Westone had a unique licensing arrangement with Sega whereby Sega retained the rights to the names "Wonder Boy" and "Monster Land", but Westone retained the rights to everything else. Westone, in turn, had a licensing arrangement with Hudson Soft, who created new characters and titles and ported these games to non-Sega platforms. The most well known of these is Adventure Island, which was more commercially successful than Wonder Boy was.