An Internet-based relapse prevention supplement to adolescent substance abuse treatment programming is a promising modality to reinforce treatment gains and enhance recovery; however, an evidence base is lacking. To assess the efficacy of the online Navigating my Journey (NmJ) program. 129 adolescent-aged participants (ages 13-23) receiving substance abuse treatment participated in a randomized parallel group study comparing two conditions: experimental (NmJ) versus attention control (viewed wellness articles from the Nemours Foundation at their discretion). Participants in the experimental condition were asked to complete 12 core lessons over 3 months. Lesson content was developed to teach evidence-based relapse prevention skills. Data were collected at four time points: baseline, 1-month follow up, 3-month follow up, and 6-month follow up. We used a linear mixed modeling approach to test for differences between conditions on each outcome. Participants in the experimental condition reported a significantly greater increase in motivation to reduce or not misuse drugs from baseline to 3-month follow up and from baseline to 6-month follow up, compared to the control participants. Participants in the experimental condition also reported a greater decrease in drug use score from baseline to 3-month follow up, compared to the control participants. An analysis of age as a potential moderator suggested that the intervention may be more effective for older adolescents. Greater use of the program was associated with greater self-efficacy and lower self-reported substance use over time. Relapse prevention treatment with adolescents may be facilitated by theory-based online interventions. NCT02125539.