In theoretical physics, a gravitational anomaly is an example of an anomaly: it is an effect of quantum mechanics - usually a one-loop diagram - that invalidates the general covariance of a theory of general relativity combined with some other fields.

The adjective "gravitational" is derived from the symmetry of a gravitational theory, namely from general covariance.

The anomaly usually appears as a Feynman diagram with a chiral fermion running in the loop (a polygon) with n external gravitons attached to the loop where n = 1 + D / 2 where D is the spacetime dimension. Anomalies only occur in even spacetime dimensions. For example, the anomalies in the usual 4 spacetime dimensions arise from triangle Feynman diagrams.

General covariance is a very important symmetry for the consistency of the whole theory, and therefore all gravitational anomalies must cancel out.