Objective: Brief interventions for adolescent marijuana users offered in schools reach users who would not otherwise present for treatment. The current study builds on previous trials of a school-based, 2-session motivational enhancement therapy (MET) intervention by adding periodic, brief motivational check-ins to reinforce gains and bolster motivation. Method: Adolescent participants were randomly assigned to a motivational check-in (MCI; n = 128) or assessment-only check-in (ACI; n = 124) comparison condition. Both conditions received 2 sessions of MET. Participants in the MCI condition then attended 3 MET-based check-in sessions at 4, 7, and 10 months after baseline, whereas the ACI condition participants attended assessment-only check-ins at the same time points. Optional cognitive behavior therapy (CBT) sessions were available as needed for 12 months from baseline in both conditions. Results: Outcomes were assessed at 6, 9, 12, and 15 months after baseline. Participants (59% Caucasian; 68% male; mean age = 15.84) reported using marijuana on an average of 37 of the last 60 days, and 70% met diagnostic criteria for a marijuana use disorder. Both conditions were associated with reduced use and negative consequences that were sustained throughout the 15 months of follow-up. The MCI condition resulted in greater reductions at the 6 months follow-up, but differences between conditions were not found at later follow-ups, and MCI participation did not increase engagement in CBT as predicted. Conclusions: Results replicated the efficacy of the brief intervention for adolescent marijuana use and provided mixed evidence on the utility of adding repeated check-in sessions.