Marvin "Grandpa" Marsh is Stan Marsh's paternal grandfather and is voiced by Trey Parker, who also provides the voices for both his son, and his grandson. He is a member of the Hare Club for Men and tries to persuade Stan to kill him on a regular basis. He made his first appearance in the Season One episode, "Death".

He has dementia, which is characterized primarily by memory loss and refers to Stan as 'Billy' as a result.