The purpose of the present study was to develop and validate a measure of confidence at self-advocacy at work (CASA-W). CASA-W is designed to assess one’s confidence at advocating for themselves in workplaces. Data were collected through Qualtrics from 200 full-time employees using Prime Panels as the recruitment platform. Results of the exploratory factor analysis indicated that the CASA-W consists of two underlying factors with a total of 18 items. The CASA-W total and subscale scores produced adequate internal consistency reliability estimates. Convergent and construct validity was supported by the relationship between CASA- W scores and measures of workplace self-efficacy (i.e., voice self-efficacy, occupational self-efficacy), proactive personality, and perceived organizational support. In terms of criterion-related validity, the CASA-W correlated as expected with measures of self-advocacy behaviors (voice, career initiative, job change negotiation) and outcomes of self-advocacy (organizational rewards growth, hierarchical status, career satisfaction). The CASA-W was also found to account for unique variance in each of the behavior and outcome variables after controlling for demographic variables, proactive personality, and perceived organizational support. Implications for theory, future research, and practice were discussed.