Friendships and romantic relationships may function as protective and risk factors for psychopathic traits. To better understand potential causal associations, we investigated whether within-individual changes in relationship characteristics were related to changes in psychopathic traits over time. Data were derived from ten repeated measurements of the Pathways to Desistance longitudinal study of 1354 offending adolescents (14.3% female; 40.1% Black). Analyses were adjusted for age, gender, ethnicity, self-reported offending and living facilities. Relationships of high quality were associated with lower psychopathic traits, whereas antisocial behavior and antisocial influence in relationships were related to higher psychopathic traits. Within-individual analysis indicated that time-invariant individual characteristics did not confound these associations. The findings suggest that the quality and antisocial activities of interpersonal relationships can affect positively or negatively on the levels of psychopathy.