OBJECTIVE: The primary objective was to evaluate the efficacy of commercially available mobile app-delivered mindfulness training (AMT), compared with waitlist control (WC), on quality of life (QOL) among women diagnosed with breast cancer. The secondary outcome was dispositional mindfulness. Enrollment, app utilization, and study completion are reported as feasibility objectives. METHODS: Women diagnosed with breast cancer ≤5 years (n = 112) were randomized to AMT (n = 57) or WC (n = 55), over 8 weeks, with 4 weeks of follow-up. We conducted linear mixed effects models to examine group by observation interactions on QOL and dispositional mindfulness at baseline, during intervention (5-weeks), post-intervention (9-weeks), and follow-up (12-weeks post-baseline). RESULTS: Participants assigned to AMT reported higher QOL, compared with those assigned to WC, from baseline through follow-up t(258.40) = 3.09, P < 0.01, 95% CI [2.71, 11.90]. Participants assigned to AMT also reported higher dispositional mindfulness, compared with those assigned to WC, from baseline through follow-up t(268.44) = 2.04, P = 0.04, 95% CI [0.01, 0.57]. App utilization data was obtained from 34 participants. Fewer participants assigned to AMT completed all study assessments, compared with participants assigned to WC, (χ2 CONCLUSIONS: Findings suggest commercially available AMT may proffer some benefit to women seeking to enhance their QOL following breast cancer diagnosis.