This study synthesizes theories of achievement motivation to better understand the development of academic task values in high school students and their relation to college major selection. We utilize longitudinal structural equation modeling to understand how grades relate to task values, how task values across domains relate to one another over time, and how the system of task values relates to college major choice. In our sample of 1,279 high students from Michigan, we find evidence that task value for math negatively relates to task value for English and vice versa. We also find that task value for math and physical science positively relates to the math-intensiveness of selected college majors, whereas English and biology task value negatively relates to math-intensiveness of majors. Gender differences in college major selection are mediated by differences in task values. Our findings have implications for theories of achievement motivation and motivational interventions.