Importance: Progress has been made in establishing evidence-based treatments for psychiatric disorders, but these are not often delivered in routine settings. A scalable solution for training clinicians in evidence-based treatments is needed. Objective: To compare 2 methods of training college (university) counseling center therapists to treat psychiatric disorders using interpersonal psychotherapy. The hypothesis was that the train-the-trainer condition would demonstrate superior implementation outcomes vs the expert condition. Moderating factors were also explored. Design, Setting, and Participants: This cluster-randomized trial was conducted from October 2012 to December 2017 in 24 college counseling centers across the United States. Therapist participants were recruited from enrolled centers, and student patients with symptoms of depression and eating disorders were recruited by therapists. Data were analyzed from 184 enrolled therapists. Interventions: Counseling centers were randomized to the expert condition, which involved a workshop and 12 months of follow-up consultation, or the train-the-trainer condition, in which a staff member from the counseling center was coached to train other staff members. Main Outcomes and Measures: The main outcome was therapist fidelity (adherence and competence) to interpersonal psychotherapy, as assessed via audio recordings of therapy sessions. Therapist knowledge of interpersonal psychotherapy was a secondary outcome. Result: A total of 184 therapists (mean [SD] age, 41.9 [10.6] years; 140 female [76.1%]; 142 white [77.2%]) were included. Both the train-the-trainer-condition and expert-condition groups showed significant within-group improvement for adherence to interpersonal psychotherapy (change: 0.233 [95% CI, 0.192-0.274] and 0.190 [0.145-0.235], respectively; both P <.001), with large effect sizes (1.64 [95% CI, 1.35-1.93] and 1.34 [95% CI, 1.02-1.66], respectively) and no significant difference between conditions. Both groups also showed significant within-group improvement in interpersonal therapy competence (change: 0.179 [95% CI, 0.132-0.226] and 0.106 [0.059-0.153], respectively; both P <.001), with a large effect size for the train-the-trainer condition (1.16 [95% CI, 0.85-1.46]; P <.001) and a significant difference between groups favoring the train-the-trainer condition (effect size, 0.47 [95% CI, 0.05-0.89]; P =.03). Knowledge of interpersonal psychotherapy improved significantly within both groups (effect sizes: train-the-trainer, 0.64 [95% CI, 0.28-0.99]; P =.005; expert, 0.69 [95% CI, 0.38-1.01]; P <.001), with no significant difference between groups. The significant moderating factors were job satisfaction for adherence (b, 0.120 [95% CI, 0.001-0.24]; P =.048) and competence (b, 0.133 [95% CI, 0.001-0.27]; P =.048), and frequency of clinical supervision for competence (b, 0.05 [95% CI, 0.004-0.09]; P =.03). Conclusions and Relevance: Results demonstrate that the train-the-trainer model produced training outcomes comparable with the expert model for adherence and was superior on competence. Given its potential capability to train more therapists over time, it has the potential to facilitate widespread dissemination of evidence-based treatments. Trial Registration: ClinicalTrials.gov Identifier: NCT02079142.