The Slender Man Mythos are the collective body of myths, stories, Creepypasta, videos, and blogs focusing on Slender Man. The original mythos were started by Victor Surge in June 2009 on the Something Awful Forums, and have since exploded and spread through the internet and sub-popular culture, especially since the creation of Marble Hornets.

Because no particularly specific canon still exists that defines the specifics of Slender Man, the mythos have evolved in a generally similar manner across separate Alternate Reality Games to all include the basics of Slender Man.