Background: Post-event processing (PEP) is an important maintenance factor of social anxiety disorder (SAD). This study examined psychometric properties of the Positive Beliefs about Post-Event Processing Questionnaire (PB-PEPQ; Fisak & Hammond, 2013), which measures metacognitive beliefs about PEP. Method: Participants receiving treatment for SAD (n = 71) and other anxiety and related disorders (n = 266) completed self-report questionnaires at several timepoints. Results: Confirmatory factor analysis did not support the PB-PEPQ's proposed unidimensional model. Subsequent exploratory factor analysis yielded a three-factor structure consisting of engaging in PEP to (1) review negative events (Negative scale), (2) review positive events (Positive scale), and (3) better understand one's social anxiety (Understand scale). Within the SAD subsample, PB-PEPQ scales demonstrated good internal consistency (α = 0.83–0.85) and test–retest reliability (r = 0.65–0.78). Convergent and criterion validity of the PB-PEPQ Negative scale were supported. PB-PEPQ scale scores were significantly higher within the SAD group, as compared with the other groups (generalised anxiety disorder, panic disorder and agoraphobia, posttraumatic stress disorder, and obsessive-compulsive disorder), supporting the scales’ discriminative validity. Conclusion: Findings support the reliability and validity of the PB-PEPQ in a clinical sample and reveal the measure's multifactorial structure.