The authors proposed the construct of workplace social self-efficacy (WSSE) and developed an inventory to measure it. Two empirical studies were conducted to examine the psychometric properties of this new measure. In Study 1, we described the development of the WSSE inventory and explored its factor structure in a sample of 304 full-time employees. Participants in Study 2 were 137 full-time employees (who provided self-report data) and 371 coworkers of these employees (who submitted peer ratings). Results showed that the WSSE inventory had a four-factor structure (social gathering, performance in public contexts, conflict management, and seeking and offering help), high internal consistency, excellent convergent and discriminant validity, and meaningful correlation patterns with related constructs in the nomological network. Furthermore, political skill was found to mediate the relationship between WSSE and several outcome variables. In addition, the WSSE inventory was found to have some advantages over the Perceived Social Self-Efficacy scale (Smith & Betz, 2000), a general social self-efficacy measure. Theoretical and practical implications were discussed.