Despite the application of Self-Efficacy Theory (Bandura, 1977, 2000) to many areas of psychology, there is a lack of research on self-efficacy in the ability to testify in court. The present study fills this gap by incrementally developing the construct of Witness Self-Efficacy and establishing its psychometric properties. Study I features exploratory and confirmatory factor analyses yielding a two-factor Witness Self-Efficacy Scale (WSES). The two components are Poise and Communication Style. Study II uses a second data collection to show that both WSES domains possess convergent, divergent, and predictive validity relations consistent with those expected using an SET framework. Notably, WSES components predicted perceptions of witness credibility and sentencing outcomes above and beyond witness extraversion, general self-efficacy, and general self-confidence. Implications for SET and witness preparation training are discussed. Copyright © 2010 John Wiley & Sons, Ltd.