This longitudinal study examined the joint role of pubertal timing and temperament variables (emotional reactivity and self-regulation) in predicting adolescents' internalizing symptoms. The multiethnic sample included 1,025 adolescent girls and boys followed from age 11 to age 15 (M age = 11.03 years at Time 1). In structural equation models, age 11 measures of pubertal timing, emotional reactivity, and self-regulation and their interactions were used to predict adolescents' internalizing behavior concurrently and at age 15. Results indicated that, among girls, early pubertal timing, higher emotional reactivity, and lower self-regulation predicted increased internalizing behavior. In addition, self-regulation moderated the effect of pubertal timing such that effects of earlier timing on subsequent internalizing were seen primarily among girls with relatively poor self-regulation. Among boys, higher levels of emotional reactivity and lower self-regulation predicted increased internalizing, but there were no effects of pubertal timing. After controlling for Time 1 internalizing symptoms, only self-regulation predicted change in internalizing symptoms. Discussion focuses on the possible interplay of temperament and pubertal development in predicting internalizing problems during adolescence.