Students' academic underachievement, defined as discrepancies between ability and academic performance, is a widespread problem leading to many negative consequences. Our study examines high school students' math underachievement, its motivational antecedents, and its impact on future math achievement, college attendance, and STEM (science, technology, engineering, and mathematics) interest. Using data from the nationally representative High School Longitudinal Study, we identified students who exhibited a math ability-achievement discrepancy between their standardized math scores at the beginning of ninth grade and their final math grade in fall of ninth grade. We then conducted generalized structural equation modeling to identify direct effects of math motivation on math underachievement along with the direct effects of math underachievement on future STEM and college outcomes. Informed by expectancy-value theory, we found that math motivation, as measured by value and expectancy beliefs was significantly associated with math underachievement. We also found that math underachievement was associated with a number of outcomes in the high school to college transition with evidence of moderation by high math ability. Implications for research, practice, and policy are discussed.