Stanley "Stan" Marsh is voiced by Trey Parker and is the most normal of the kids, and is generally honest and well-meaning. He is somewhat more assertive than his best friend Kyle. Stan is also notably the skeptic of the group, and is often preachy and very critical of popular trends.

He is the son of Randall "Randy" Marsh, a geologist with the United States Geological Survey, and Sharon Marsh, a receptionist at Tom's Rhinoplasty. He has a sister, Shelley, named after Trey Parker's sister, who beats him up on a regular basis and often calls him a "turd", and a gay dog named Sparky.