Temperament and externalizing problems are closely linked, but research on how they codevelop across adolescence remains sparse and equivocal. Reinforcement sensitivity theory (RST) provides a useful framework for understanding temperament and externalizing problems associations. During adolescence, oppositional problems are posited to be linked to an overactive behavioral approach system (BAS) while conduct problems are linked to an underactive behavioral inhibition system (BIS). However, this research mostly uses adult samples and cross-sectional designs and tests only between-person associations. Moreover, most studies typically test only one direction of effects (i.e., temperament predicts externalizing problems) and do not consider alternative models of associations, such as reciprocal associations. To address these limitations, we use three annual waves of a longitudinal, community-based sample of 387 early adolescents (mean Wave 1 age = 11.61 years, 55% female, 83% non-Hispanic White) to test reciprocal associations between BIS and BAS and oppositional and conduct problems. Latent curve models with structured residuals are used to test hypotheses and disaggregate between- and within-person associations. Evidence supports within-person reciprocal associations between BAS and oppositional problems and between BIS and combined conduct and oppositional problems. Results potentially inform developmental theories of temperament and externalizing problems linkages and interventions with adolescents who are engaging in oppositional problems and more severe conduct problems.