The purpose of this study is to provide evidence of reliability and validity of the Self-Efficacy for Teaching Mathematics Instrument (SETMI). Self-efficacy, as defined by Bandura, was the theoretical framework for the development of the instrument. The complex belief systems of mathematics teachers, as touted by Ernest provided insights into the elements of mathematics beliefs that could be relative to a teacher’s self-efficacy beliefs. The SETMI was developed in July 2010 and has undergone revisions to the original version through processes defined in this study. Evidence of reliability and validity were collected to determine whether the SETMI is an adequate instrument to measure self-efficacy of elementary mathematics teachers. Construct validity of the revised SETMI was tested using confirmatory factor analysis. Findings indicate that the SETMI is a valid and reliable measure of two aspects of self-efficacy: pedagogy in mathematics and teaching mathematics content.