A continual rise in the prevalence of depression underscores the need for effective, long-lasting interventions that can be widely accessed. To address this, we created a comprehensive, smartphone-based, 8-week digital intervention comprising several evidence-based practices for adults with depression. Recently, there has been substantial support for heart rate variability biofeedback (HRV-B) as a primary or adjunct treatment for several mental health disorders, including depression. The objectives of this study were to determine if the addition of HRV-B to our original intervention was feasible and whether patients in the HRV-B (“enhanced”) intervention were more likely to experience clinically significant improvements in depressive symptoms than patients in our original (“standard”) intervention. In this quasi-experimental, matched subjects study, patients in the enhanced group (n = 48) completed a total average of 3.86 hours of HRV-B practice across 25.8 sessions, and were more likely to report a clinically significant improvement in PHQ-9 score post-intervention than participants in the standard group (n = 48), even after adjusting for differences in demographics and engagement between groups (adjusted OR = 3.44, 95% CI [1.28 – 9.26], p = 0.015). Our findings suggest that adding HRV-B to an app-based, smartphone-delivered, remote intervention for depression is feasible and may enhance treatment outcomes.