The evil demon, also known as Deus deceptor, malicious demon, and evil genius, is an epistemological concept that features prominently in Cartesian philosophy. In the first of his 1641 Meditations on First Philosophy, Descartes imagines that a malevolent God or an evil demon, of "utmost power and cunning has employed all his energies in order to deceive me." This malevolent God or evil demon is imagined to present a complete illusion of an external world, so that Descartes can say, "I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things."

Some Cartesian scholars opine that the malevolent God or evil demon is also omnipotent, and thus capable of altering mathematics and the fundamentals of logic, though omnipotence of the malevolent God or evil demon would be contrary to Descartes' hypothesis, as he rebuked accusations of the evil demon having omnipotence. It is one of several methods of systematic doubt that Descartes employs in the Meditations.