Gruel is a food consisting of some type of cereal—such as ground oats, wheat, rye, or rice—heated or boiled in water or milk. It is a thinner version of porridge that may be more often drunk rather than eaten. Historically, gruel has been a staple of the Western diet, especially for peasants. Gruel may be made from millet, hemp, barley, or, in hard times, from chestnut flour or even the less bitter acorns of some oaks. Gruel has historically been associated with feeding the sick and recently weaned children.

Gruel is also a colloquial expression for any watery food of unknown character, e.g., pea soup. Gruel has often been associated with poverty, with negative associations attached to the term in popular culture, as in the Charles Dickens novels Oliver Twist and A Christmas Carol.