The Tulpa Effect is a theory on the creation and existence of Slender Man.

A Tulpa is a thoughtform, or being created from the collective thoughts of separate individuals. Tulpa are theoretical in nature, although some semi-scientific research and studies were conducted in the area between 1960-1980. The Tulpa Effect is the name given to the unintentional creation of a tulpa based on collective belief of a being with similar traits.

In relation to Slender Man, the idea of intentional creation of Slender Man as a tulpa came as early as August of 2009, as the idea was first thrown around in the Something Awful forums. The idea is still carried today by some Slender Man-based communities, specifically Slender Nation, although the separate evolutions of the Slender Man creature has created varying beliefs, traits, descriptions, and actions, and no longer includes a single unified description of Slender Man from which a tulpa could be created.

Some have worries over the unintentional creation of a Slender woman tulpa due to the popularity of the mythos in present day. Core Theory centers around the idea that the Slender Man was created as a tulpa in some way in the various ARGs, and acts as an in-game reasoning as to its existence and slight differences among the various series. Others believe that the Tulpa Effect may have serious real-life repercussions with the creation of a real Slender Man.

A competing theory of Slender Man's origin is Quantum Theory.