Insufficient research attention has been focused on the development of academic "under"-achievement and motivational beliefs. A longitudinal, person-centered approach was used to identify latent subgroups of growth in the joint development of teacher-reported underachievement and four motivational beliefs (self-concept, task importance, psychological cost value, and self-worth) from first through sixth grade. Two types of underachievement emerged: Sustained and Growing Underachievement. An Achievement class was also consistently found. Sustained underachievement was not associated with declining self-concept or task importance, but was related to moderately lower psychological cost value, self-worth, and middle school achievement. Growing underachievement was associated with relatively lower levels of self-concept and task importance that declined over time, similar to the achievement patterns. Class membership did not differ by gifted status. Gender effects emerged for the Task Importance and Self-Worth models in the hypothesized direction, but were not robust. These findings highlight heterogeneity in the development of academic underachievement.