IMPORTANCE: Employer-sponsored benefit programs aim to increase access to behavioral health care, which may help contain health care costs. However, research has either focused solely on clinical outcomes or demonstrated reductions in medical claims without accounting for the costs of behavioral health services, leaving the financial return on investment unknown. OBJECTIVE: To determine whether a clinically effective employer-sponsored behavioral health benefit is associated with net medical cost savings. DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study included participants eligible for an employer-sponsored behavioral health benefit between November 1, 2019, and May 31, 2023. Eligibility criteria included having a behavioral health diagnosis and, in the program group, attending at least 1 behavioral health appointment. Program users were matched to nonusers on medical risk scores, behavioral health diagnoses, date of diagnosis, age, sex, and employer. Participants were followed up for 1 year before and after the benefit launch. EXPOSURE: A digital platform screened individuals for common behavioral health conditions and provided access to video and in-person psychotherapy, medication management, care navigation, and self-guided digital content. MAIN OUTCOMES AND MEASURES: Primary outcomes were per member per month (PMPM) medical spending, inclusive of all medical claims and program costs. A difference-in-differences analysis was used to compare changes in net medical spending between groups from the year before and up to 1 year after an index mental health diagnosis. RESULTS: This study included 13 990 participants: 4907 of 4949 (99.1%) eligible program group members were matched to 9083 control participants. Their mean (SD) age was 37 (13.2) years, and most participants (65.5%) were female. Costs decreased in the program group relative to the control group, with a net difference-in-differences of -64 PMPM (95% CI, -28 to -00 PMPM), corresponding to savings of 070 per participant in the first program year and a return on investment of 1.9 times the costs (ie, every 00 invested reduced medical claims costs by 90). Behavioral health costs in the program group increased relative to the control group but were more than offset by decreases in physical health care costs. Savings were larger for participants with higher medical risk. CONCLUSIONS AND RELEVANCE: In this cohort study, every 00 invested in an employer-sponsored behavioral health program with fast access to psychotherapy and medication management was associated with a reduction in medical claims costs by 90. These findings suggest that expanding access to behavioral health care may be a financially viable cost-reduction strategy for health care buyers.