The present study used a Self-Efficacy Theory (SET; Bandura, 1977, 1986) framework to address the need for an outcome measure in witness preparation training. The construct of Witness Self-Efficacy, broadly defined as a witness's perceived ability to testify in court, was developed drawing largely on existing self-efficacy literature. The goal of the study was to establish initial psychometric properties for the Witness Self-Efficacy Scale (WSES). Participants acting either as mock witnesses or jurors took part in a two-phase study. First, 41 mock witnesses were asked to provide a written account of being falsely accused of an act. They were then briefly deposed about the account, and completed the WSES and construct validity questionnaires. Mock witnesses finally testified under cross-examination from a mock attorney about the accusations. In the second portion, a total of 290 mock jurors (six to eight per group) observed videotaped testimonies of participants from phase one. Mock jurors then provided predictive validity ratings of witness credibility, believability, innocence likelihood, efficacy, and agreement with testimony. Bi-variate correlation analyses showed that the WSES was significantly positively related with general self-efficacy, social self-efficacy, and general self-confidence. The WSES was significantly negatively related to introversion. The scale displayed non-significant relations with social desirability, depression, self-esteem, and innocence expectancy. Multivariate regression analyses showed that the scale failed to predict any of the primary mock juror ratings. Follow-up multivariate regression analyses showed that mock juror ratings of witness confidence and the WSES predicted dependent measures, although these independent variables were collinear. Finally, there was an interaction between self and observer ratings of the WSES as it predicted witness credibility. Results are discussed with regard to theoretical and applied implications. Contrary to SET, self-reported WSE appears to be largely correlated with confidence and fails to predict outcomes on its own. Concerning witness preparation, the WSES possesses solid construct validity and reliability, but requires further testing within a witness preparation training model to assess its practical utility. A self-efficacy based method of witness preparation is proposed as a manner of continued research using the WSES.