Given the importance of mathematics identity for students continued participation and engagement with mathematics, it is important for educators and researchers to be able to explore students' mathematics identity development. However, an instrument with validity evidence that can be used to explore mathematics identity efficiently and with groups of students is not currently available. This article draws on prior research to test and validate items for a mathematics identity instrument to be used with fifth through twelfth grade students. This study includes 1559 participants from two school districts in a midwestern state in the USA. Analysis includes assessing four components of validity including content validity, internal structure, relationship to other variables, and generalization. Findings provide evidence for the reliability and validity of the items in the mathematics identity scale, giving researchers and educators a way to explore this construct efficiently.