The current study investigated whether prosocial behavior and emotional problems, peer problems, conduct problems, and hyperactivity and inattention problems were long-term longitudinally and bidirectionally related at inter- and or intra-individual levels from early childhood through mid-adolescence. Parents in the United Kingdom reported their child's prosocial behavior and multidimensional psychopathology at ages 3, 5, 7, 11, and 14 years (N = 16,984, 51% male, 83% White). Four random intercepts cross-lagged panel models were fitted. Higher levels of earlier prosocial behavior were associated with greater than expected decrements in psychopathology. At an intraindividual, within-person level, prosocial behavior was negatively bidirectionally associated with peer, conduct, and hyperactivity and inattention problems. Also at an intraindividual, within-person level, prosocial behavior was unidirectionally protective against emotional problems. At an interindividual level, prosocial behavior and each dimension of psychopathology were negatively associated. Therefore, engaging in prosocial behavior can reduce psychopathological symptoms over time (and vice versa), and youth who are more prosocial also tend to experience fewer psychopathological symptoms. Intraindividual associations were small while interindividual associations were moderate to large. Implications for theory, future research, and evidence-based interventions are discussed.