Summary: STEM-related industries are a major driver of U.S. economic growth and possessing mathematics skills is a central component of success in STEM fields. Thus, it is important to identify predictors of mathematics achievement in high school students. Some previous research has shown that studentsâ€™ beliefs and attitudes are important predictors of their mathematics grades and achievement. In the present paper, we examined the ability of mathematics beliefs and attitudes to predict ACT mathematics test scores by analyzing data from a questionnaire designed using the Theory of Planned Behavior framework given to high school juniors and seniors who had recently taken the ACT. Results revealed that mathematics attitudes, subjective norms, perceived control, and intentions were all significantly correlated with mathematics course grades and ACT mathematics scores. Attitudes also incrementally predicted ACT mathematics scores over several key variables. The article concludes with a discussion of limitations and future directions.