Wonder Boy in Monster Land, known by its original arcade release as Wonder Boy: Monster Land, is an platform video game developed by Westone Bit Entertainment and released by Sega in Japanese arcades in 1987 and for the Master System in 1988, with a number of other home computer and console ports following. The game is the sequel to the 1986 game Wonder Boy and takes place eleven years after the events in the previous game. After enjoying over a decade of peace on Wonder Land following the defeat of the evil King by Tom-Tom, later bestowed the title "Wonder Boy", a fire-breathing dragon called the MEKA dragon appeared; he and his minions conquered Wonder Land, turning it into "Monster Land". The people, helpless due to their lack of fighting skill, call for Wonder Boy, now a teenager, to destroy the monsters and defeat the MEKA dragon. Players control Wonder Boy through twelve linear levels as he makes his way through Monster Land to find and defeat the MEKA dragon. Players earn gold by defeating enemies and buy weapons, armor, footwear, magic, and other items to help along the way.

The arcade version of Wonder Boy in Monster Land amassed moderate sales, the Master System version received overall positive reviews in all aspects and has been highly regarded as one of the better titles in the Master System library. Reviews praised the game for its colorful graphics, smooth controls, gameplay, and replay value. Other ports received mixed reception: criticisms included very slow multi-load times on the home computer versions, sub-quality sound, and smaller play areas. The arcade version would later be re-released in emulated form as a digital download for Wii, PlayStation 3 and Xbox 360. It had a sequel, Wonder Boy III: The Dragon's Trap, released for the Master System in 1989.