NetHack is an open source single-player roguelike video game, first released in 1987 and maintained by the NetHack DevTeam. The game is a fork of the 1982 game Hack, itself inspired by the 1980 game Rogue. The player takes the role of one of several pre-defined character classes to descend through multiple dungeon floors, fighting monsters and collecting treasure, to recover the "Amulet of Yendor" at the lowest floor and then escape.

As an exemplar of the traditional "roguelike" game, NetHack features turn-based, grid-based hack and slash dungeon crawling gameplay, procedurally generated dungeons and treasure, and permadeath, requiring the player to restart the game anew should the player character die. The game uses simple ASCII graphics by default so as to display readily on a wide variety of computer displays, but can use curses with box-drawing characters, as well as substitute graphical tilesets on machines with graphics. While Rogue, Hack and other earlier roguelikes stayed true to a high fantasy setting, NetHack introduced humorous and anachronistic elements over time, including popular cultural reference to works such as Discworld and Raiders of the Lost Ark.

It is identified as one of the "major roguelikes" by John Harris. Comparing it with Rogue, Engadget's Justin Olivetti wrote that it took its exploration aspect and "made it far richer with an encyclopedia of objects, a larger vocabulary, a wealth of pop culture mentions, and a puzzler's attitude." In 2000, Salon described it as "one of the finest gaming experiences the computing world has to offer".