Teachers’ self-efficacy has been measured a number of ways and linked to important outcomes including teacher satisfaction, goals for students, and student achievement. However, most of these existing measures are intentionally general in their design. Given the importance of context, researchers have begun to construct more domain-specific measures of teacher self-efficacy. In this article, we describe the process by which we modified an existing instrument to measure mathematics teachers’ self-efficacy beliefs, and the evidence for the validity and reliability of the modified instrument, the Mathematics Teacher Sense of Efficacy Scale (MTSES). This validation work will enable other researchers and practitioners to use the MTSES to measure mathematics teachers’ self-efficacy beliefs.