The purpose of this study was to determine whether motivational interviewing (MI), compared with an attention control condition (relaxation training [RT]) enhances substance abuse treatment engagement in incarcerated adolescents. At the start of incarceration, adolescents were randomly assigned to individually administered MI or RT. Subsequently, therapists and adolescents (N = 130) rated degree of adolescent participation in the facility's standard care group-based treatments targeting crime and substance use. All adolescents received the facility standard care treatment after their individual MI or RT session. MI statistically significantly mitigated negative substance abuse treatment engagement. Other indicators of treatment engagement were in the expected direction; however, effect sizes were small and nonsignificant. These findings are significant, given concerns regarding the deleterious effects of treating delinquent adolescents in groups and the potential for adolescents to reinforce each other's negative behavior, which in turn may lead to escalated substance use and other delinquent behaviors after release.