【Background】Peer-supported mobile health (mHealth) programs hold the promise of providing a low-burden approach to increasing access to care and improving mental health. While peer support has been shown to improve engagement in care, there is limited investigation into the impact of peers on symptom outcomes. Trauma-exposed populations frequently endure co-occurring posttraumatic stress and depressive symptoms as well as difficulties in day-to-day functioning. This study evaluated the potential benefits of a peer-supported, transdiagnostic mHealth program on symptom outcomes and functioning.【Objective】This randomized controlled trial tested the effectiveness of Brief Peer-Supported (BPS) web-based Skills Training in Affective and Interpersonal Regulation (webSTAIR), a 6-module transdiagnostic digital program derived from Skills Training in Affective and Interpersonal Regulation and compared to waitlist control in a community sample of veterans who screened positive for either posttraumatic stress disorder (PTSD) or depression.【Methods】A total of 178 veterans were enrolled in this study using a 2:1 randomization scheme with 117 assigned to BPS webSTAIR and 61 assigned to waitlist control. PTSD and depressive symptoms as well as emotion regulation and psychosocial functioning were assessed at pretreatment, posttreatment, and 8-week follow-up time points. Mixed-effects models were used to assess change in outcome measures across time points. Exploratory analyses were conducted to determine whether the type and number of peer interactions influenced outcomes.【Results】Significant interaction effects were observed for all outcomes such that participants randomized to BPS webSTAIR reported significantly greater improvement at the posttreatment time point compared to waitlist control with moderate effect sizes for PTSD (d=0.48), depression (d=0.64), emotion regulation (d=0.61), and functional impairment (d=0.61); gains were maintained at 8-week follow-up. An initial cohort of participants who were required to engage with a peer coach to progress through the modules interacted more frequently with peers but completed fewer modules compared to a later cohort for whom peer engagement was optional. Overall, those who completed more modules reported greater improvement in all outcomes.【Conclusions】BPS webSTAIR was effective in improving PTSD and depression symptoms, emotion regulation, and psychosocial functioning in community veterans. Peer-supported, transdiagnostic mHealth programs may be a particularly efficient, effective, and low-burden approach to improving mental health among trauma-exposed populations. Investigation of peer-supported programs among other populations is necessary to evaluate the generalizability of the findings. Analyses comparing peer support that was required versus optional indicated that some veterans may not need or want peer support. Future research should evaluate how best to deliver peer support and for whom it is most beneficial. If successful, peer-supported tech programs may increase the Veteran Affairs workforce as well as improve veteran mental health services and outcomes.【Trial registration】ClinicalTrials.gov NCT04286165; https://clinicaltrials.gov/study/NCT04286165.