Background: Diaphragmatic breathing is an evidence-based intervention for managing stress and anxiety; however, some military veterans with mild traumatic brain injury (mTBI) and posttraumatic stress disorder (PTSD) report challenges to learning and practicing the technique. BreatheWell Wear assists performance of breathing exercises through reminders, biofeedback, and visual, tactile, and auditory guidance. Objective: To evaluate feasibility of implementing BreatheWell Wear, a mobile smartwatch application with companion smartphone app, as an intervention for stress management in military veterans with mTBI and PTSD. Methods: Thirty veterans with chronic symptoms of mTBI and PTSD recruited from an interdisciplinary, intensive outpatient program participated in this pilot pragmatic clinical trial. Participants were randomly assigned to the experimental (BreatheWell Wear and conventional care) and control (conventional care) groups for 4 weeks. Conventional care included instruction on relaxation breathing and participation in behavioral health therapy. Effects on goal attainment, treatment adherence, diaphragmatic breathing technique knowledge, and stress were measured through surveys and diaries. Changes in symptoms, mood, and well-being were measured pre/postintervention via the Posttraumatic Checklist for DSM-5, Beck Anxiety Inventory, Beck Depression Inventory, and Flourishing Scale. Results: Person-centered goal attainment (t = 4.009, P < .001), treatment adherence (t = 2.742, P = .001), diaphragmatic breathing technique knowledge (t = 1.637, P < .001), and reported ease of remembering to practice (t = −3.075, P = .005) were significantly greater in the experimental group. As expected, measures of PTSD, anxiety, depression, and psychological well-being showed clinically meaningful change in both groups, and both groups demonstrated reduced stress following diaphragmatic breathing. Conclusion: These preliminary findings indicate that BreatheWell Wear may be a clinically feasible tool for supporting diaphragmatic breathing as an intervention in veterans with mTBI and PTSD, and a future effectiveness trial is warranted.