Patients’ perception of treatment credibility represents their belief about a treatment’s personal logicality, suitability, and efficaciousness. Although long considered an important common factor bearing on clinical outcome, there have been no systematic reviews of the credibility–outcome association. The present study represents a meta-analysis of the association between patients’ credibility perception and their posttreatment outcomes. To be included, articles published through August, 2017 had to (a) include a clinical sample, (b) include a therapist-delivered treatment of at least 3 sessions, (c) include a measure of patients’ own early treatment credibility perception, (d) include at least 1 posttreatment mental health outcome not explicitly referenced as a follow-up occasion, and (e) report a statistical test of the credibility–outcome association. The meta-analysis was conducted on 24 independent samples (extracted from 19 references) with 1,504 patients. The overall weighted effect size was r = .12, p < .001, or d = .24, with high heterogeneity (I2 = 57%) and no evidence of publication bias. There were no significant moderating effects on the credibility–outcome association for any of the potential moderators that we evaluated. The meta-analytic findings are discussed in light of methodological limitations and with regard to their practice implications.