Zork is a text-based adventure game first released in 1977 by developers Tim Anderson, Marc Blank, Bruce Daniels, and Dave Lebling for the PDP-10 mainframe computer. The original developers and others, as the company Infocom, expanded and split the game into three titles—Zork I: The Great Underground Empire, Zork II: The Wizard of Frobozz, and Zork III: The Dungeon Master—which were released commercially for a range of personal computers beginning in 1980. In Zork, the player explores the abandoned Great Underground Empire in search of treasure. The player moves between the game's hundreds of locations and interacts with objects by typing commands in natural language that the game interprets. The program acts as a narrator, describing the player's location and the results of the player's commands. It has been described as the most famous piece of interactive fiction.

The original game, developed between 1977 and 1979 at the Massachusetts Institute of Technology (MIT), was inspired by Colossal Cave Adventure (1976), the first well-known example of interactive fiction and the first well-known adventure game. The developers wanted to make a similar game that was able to understand more complicated sentences than Adventure's two-word commands. In 1979, they founded Infocom with several other colleagues at the MIT computer center. Blank and Joel Berez created a way to run a smaller portion of Zork on several brands of microcomputer, letting them commercialize the game as Infocom's first products. The first episode was published by Personal Software in 1980, after which Infocom purchased back the rights and self-published all three episodes beginning in late 1981.

Zork was a massive success for Infocom, with sales increasing for years as the market for personal computers expanded. The first episode sold more than 38,000 copies in 1982, and around 150,000 copies in 1984. Collectively, the three episodes sold more than 680,000 copies through 1986, comprising more than one-third of Infocom's sales in this period. Infocom was purchased by Activision in 1986, leading to new Zork games beginning in 1987, as well as a series of books. Reviews of the episodes were very positive, several reviewers calling Zork the best adventure game to date. Critics regard it as one of the greatest video games. Later historians have noted the game as foundational to the adventure game genre, as well as influencing the MUD and massively multiplayer online role-playing game genres. In 2007, Zork was included in the game canon by the Library of Congress as one of the ten most important video games in history.