An alternate reality game (ARG) is an interactive narrative that uses the real world as a platform and uses transmedia storytelling to deliver a story that may be altered by participants' ideas or actions.

The form is defined by intense player involvement with a story that takes place in real-time and evolves according to participants' responses. Subsequently, it is shaped by characters that are actively controlled by the game's designers, as opposed to being controlled by artificial intelligence as in a computer or console video game. Players interact directly with characters in the game, solve plot-based challenges and puzzles, and collaborate as a community to analyze the story and coordinate real-life and online activities. ARGs generally use multimedia, such as telephones, email and mail but rely on the Internet as the central binding medium.

ARGs are growing in popularity, with new games appearing regularly and an increasing amount of experimentation with new models and subgenres. They tend to be free to play, with costs absorbed either through supporting products (e.g. collectible puzzle cards fund Perplex City) or through promotional relationships with existing products (for example, I Love Bees was a promotion for Halo 2, and the Lost Experience and Find 815 promoted the television show Lost). However, pay-to-play models are not unheard of.