Approximately 20% of women suffer from postpartum depression (PPD). Due to barriers such as limited access to care, half of the women with PPD do not receive treatment. Therefore, it is critical to identify effective and scalable interventions. Traditional mindfulness programs have been effective in reducing depressive symptoms, however access remains a barrier. A self-paced mobile health (mHealth) mindfulness program may fit the lifestyle of busy mothers who are unable to attend in-person classes. However, little is known regarding the feasibility or efficacy of mHealth mindfulness interventions in postpartum women with depressive symptoms. This study aims to assess the feasibility, acceptability, and preliminary efficacy of an mHealth mindfulness intervention for postpartum women with moderate to moderately severe depressive symptoms. We conducted a single-arm feasibility trial of an mHealth mindfulness intervention within Kaiser Permanente Northern California (KPNC), a large integrated health care system. Participants were identified through clinician referral and electronic health records via KPNC’s universal perinatal depression screening program and recruited by the study team. Inclusion criteria included the following: English-speaking, up to 6 months postpartum with a Patient Health Questionnaire (PHQ-8) score of 10 to 19, and no regular mindfulness/meditation practice. Participants were asked to use a mindfulness app, Headspace, 10 to 20 min/day for 6 weeks. Baseline and postintervention surveys captured data on patient-reported outcomes (depression and stress symptoms, sleep quality, and mindfulness). Semistructured interviews captured acceptability. Retention and adherence were used to assess feasibility. Of the 115 women who were contacted and met the eligibility criteria or declined participation before eligibility assessment, 27 (23%) were enrolled. In addition, 70% (19/27) completed the study. The mean age of participants was 31 years (SD 5.2), 30% (8/27) were non-Hispanic White, and, on average, participants were 12.3 weeks postpartum (SD 5.7). Of the women who completed the study, 100% (19/19) used the Headspace app at least once, and nearly half (9/19, 47%) used the app on ≥50% of the days during the 6-week intervention period. Of the 16 participants who completed the postintervention interview, 69% (11/16) reported that they were very or extremely satisfied with the app. Interviews indicated that women appreciated the variety of meditations and felt that the program led to reduced anxiety and improved sleep. Significant improvements in pre- and postintervention scores were observed for depressive symptoms (PHQ-8: -3.8, P=.004), perceived stress (10-item Perceived Stress Scale: -6.0, P=.005), and sleep quality (Pittsburgh Sleep Quality Index: -2.1, P=.02, indicating less sleep disturbance). Improvements in mindfulness were also significant (Five Facet Mindfulness Questionnaire-Short Form: 10.9, P=.01). An mHealth mindfulness intervention for postpartum women with moderate to moderately severe depressive symptoms is feasible and acceptable. An efficacy trial is warranted.