The Sinister Secret of Saltmarsh is a module for the Advanced Dungeons & Dragons (AD&D) roleplaying game, written by Dave J. Browne with Don Turnbull. The module details a mysterious abandoned mansion at the edge of a town called Saltmarsh, and the secrets contained therein. The adventure is set in the World of Greyhawk campaign setting. The Sinister Secret of Saltmarsh received positive reviews from critics.

Greyhawk, also known as the World of Greyhawk, is a fictional world designed as a campaign setting for the Dungeons & Dragons fantasy roleplaying game. Although not the first campaign world developed for Dungeons & Dragons—Dave Arneson's Blackmoor campaign predated it by about a year—the world of Greyhawk closely identified with early development of the game beginning in 1972, and after being published it remained associated with Dungeons & Dragons publications until 2008. The world itself started as a simple dungeon under a castle designed by Gary Gygax for the amusement of his children and friends, but it was rapidly expanded to include not only a complex multi-layered dungeon environment, but also the nearby city of Greyhawk, and eventually an entire world. In addition to the campaign world, which was published in several editions over twenty years, Greyhawk was also used as the setting for many adventures published in support of the game, as well as for RPGA's massively shared Living Greyhawk campaign from 2000 to 2008.