In the current article, we describe the development and validation of the Mistake Rumination Scale as a supplement to existing trait and cognitive measures of perfectionism. The Mistake Rumination Scale is a seven-item inventory that taps the tendency to ruminate about a past personal mistake. Psychometric analyses confirmed that the Mistake Rumination Scale consists of one factor assessed with high internal consistency. Analyses established that elevated levels of mistake rumination are associated with trait perfectionism, ruminative brooding, depression, and social anxiety. Moreover, scores on the Mistake Rumination Scale significantly predicted unique variance in depression beyond the variance accounted for by trait perfectionism dimensions, rumination, and automatic thoughts. Overall, our findings attest to the further use of the Mistake Rumination Scale and highlight the tendency of certain perfectionists to suffer from cognitive forms of perfectionism involving overthinking about past mistakes and related imperfections involving the self.