One important way in which people assert their agency in the workplace is by engaging in self-advocacy. We used the social cognitive model of career self-management (CSM; Lent & Brown, 2013) to examine hypothesized predictors and outcomes of workers’ engagement in self-advocacy. Participants were 511 full-time employees who completed an online survey of self-assertive efficacy involving advocacy behavior, proactive personality, perceived organizational support, self-advocacy behaviors (voice, career initiative, job change negotiation), and three career success criteria (career satisfaction, organizational rewards growth, and hierarchical status at work). Splitting the sample into two parts, we first examined the psychometric properties of a new measure of self-assertive efficacy in relation to workplace advocacy behaviors (SAE-W; n = 200). In the second phase, we tested a set of measurement models to confirm the factor structure of the new measure (n = 311). Results suggested that the SAE-W measure can be viewed as largely unidimensional. Path analyses of the CSM model, including the new self-efficacy measure, offered good fit to the data across gender, racial/ethnic groups, and educational level. We discuss the implications of the findings for future research on self-advocacy behaviors in the workplace from a social cognitive perspective.