In contract law, an illusory promise is one that courts will not enforce. Contrast with a contract, which is a promise that courts will enforce. A promise may be illusory for a number of reasons. In common law countries this usually results from failure or lack of consideration.

Illusory promises are so named because they merely hold the illusion of contract. For example, a promise of the form, "I will give you ten dollars if I feel like it." is purely illusory and will not be enforced as a contract.