BACKGROUND: Depressed young women have elevated rates of unintended pregnancy and sexually transmitted infections (STIs). The objective of this study was to develop and pilot-test a counseling-plus-mHealth intervention to reduce sexual and reproductive health (SRH) risk in young women with depressive symptoms. METHODS: Using the Behavior-Determinants-Intervention logic model, we developed the Momentary Affect Regulation-Safer Sex Intervention (MARSSI) to address the challenges that depression imposes on SRH risk reduction efforts of high-risk young women: (I) in-person counseling using motivational interviewing (MI) to elicit motivation for safer sex and develop a behavior change plan, and teaching cognitive-behavioral skills to manage negative thoughts and affective states; (II) 4-week Ecological Momentary Intervention (EMI) on a smartphone to report momentary phenomena related to depression and SRH risk, and receive personalized, tailored messages prompting healthy behaviors and encouraging cognitive-behavioral skill use when risk-related cognitions and negative affect are reported; and (III) booster counseling to review behavior change goals and plans and teach a new cognitive-behavioral skill. We developed the counseling through iterative interviews with 11 participants and developed the EMI through a 2-week trial with three participants, then revised MARSSI to reflect participant feedback. We next conducted a pilot-test among depressed, high-risk female adolescent clinic patients age 15-24. Pilot participants completed mental health, motivation to change behavior, and SRH behavior assessments and provided feedback at baseline, post-EMI, and at 3-month follow-up. We analyzed participant retention, counseling duration, app engagement, intervention quality ratings, and participant feedback, and compared mental health and SRH risk behavior across the study. RESULTS: Seventeen participants completed the initial counseling session, 15 participated in the EMI, 14 returned for the booster session, and 14 completed the 3-month follow-up. App engagement was high for all 4 EMI weeks (≥1 report/day for median ≥6 days/week). Post-intervention, most or all participants agreed with each positive statement about the messages, reported "Excellent" MARSSI usefulness, and attributed improvements to MARSSI. Compared to baseline, post-EMI depressive symptoms, confidence to change self-selected risk behavior, and confidence to use the cognitive restructuring skill improved. At 3 months, depressive symptom scores remained lower and confidence to use cognitive restructuring remained higher, compared to baseline. Participants also reported lower frequency of sex, lower proportion of condom-unprotected sex events, and, among those using effective contraception, more consistent condom use at 3-month follow-up vs. baseline. CONCLUSIONS: MARSSI was feasible, acceptable, and engaging to young women with depression and SRH risk behavior, and was associated with increased confidence to reduce SRH risk, decreased SRH risk behaviors, increased confidence to use cognitive restructuring, and decreased depressive symptoms over 3 months. Future research is warranted to evaluate MARSSI's efficacy to improve motivation, skills, affect, and behaviors, as well as reproductive health outcomes in high-risk depressed young women.