The Slender Man (also known as Slenderman) is a supernatural creature with nebulously defined characteristics and abilities. Slender Man was first mentioned in Something Awful Forum's "Create Paranormal Images". He generally appears (in modern times) as a tall humanoid creature in a black or grey suit, red or black tie, and white shirt. His face is totally white, completely devoid of facial features. He has no hair, and generally has normal-looking bare hands, albeit with fingers longer and bonier than a typical human. Slender Man has been depicted in imagery and literature at anywhere between 6 and 15 feet tall, depending on the situation, though in video he is usually only around 6-7 feet tall. Because of his inexact nature, and differences between accounts, no one has yet determined what, exactly, the Slender Man is. Currently, there are two leading theories as to what the Slender Man may be: the Tulpa Effect.

The internet popularity of Slender Man has spawned a massive number of interconnected ARGs, collectively known as the Slender Man Mythos.