Postpartum depression (PPD) is associated with significant health consequences for the parent and child. Current recommendations for PPD prevention require intense health care system resources. Evidence-based interventions for PPD prevention that do not further burden the health care system are needed. Evidence suggests that physical activity (PA) can generally reduce depressive symptoms. Technology-based interventions may help decrease common barriers to PA. This study aims to report the protocol and provide a data overview of the POstpartum Wellness study (POW)-an effectiveness trial evaluating whether an eHealth PA intervention tailored for postpartum individuals increased PA and decreased depressive symptoms among individuals at high PPD risk. This remote parallel-group randomized controlled trial included postpartum individuals with a history of depression or at least moderate current depressive symptoms not meeting the PPD diagnostic threshold and with low PA levels from an integrated health care delivery system. Participants were randomized to an eHealth PA intervention or usual care. The intervention group received access to a library of web-based workout videos designed for postpartum individuals, which included interaction with their infants. At baseline and follow-up (3 and 6 months), PA was measured using questionnaires and a wrist-worn accelerometer. Depressive symptoms were measured using the Patient Health Questionnaire-8 (PHQ-8). Data were collected to assess exploratory outcomes of sleep, perceived stress, anxiety, parent-infant bonding, and infant development. The study was funded in January 2020. Participants were enrolled via REDCap (Research Electronic Data Capture) or telephonically between November 2020 and September 2022; data collection ended in April 2023. Randomized participants (N=99) were 4 months post partum at baseline with moderately severe depressive symptoms (mean PHQ-8 score 12.6, SD 2.2). Intervention (n=50) and usual care (n=49) groups had similar sociodemographic characteristics, months post partum, baseline depressive symptoms, number of children at home, and prepregnancy PA levels. Retention in assessments was ≥66% for questionnaires and ≥48% for accelerometry, with modest differences by group. At 3-month follow-up, 73 of 99 (74%) participants (intervention: 35/50, 70%; usual care: 38/49, 78%) completed questionnaires; 53 of 99 (54%) wore the accelerometer for 7 days (27 of 50 (54%) intervention, 26 of 49 (53%) usual care). At 6-month follow-up, 66 of 99 (67%) participants (30 of 50 (60%) intervention, 36 of 49 (73%) usual care) completed questionnaires and 43 of 99 (43%) wore the accelerometer for 7 days (21 of 50 (42%) intervention, 22 of 49 (45%) usual care). Data analysis is completed, and a manuscript with these findings is currently under review for publication. The POW trial evaluates the effectiveness of an eHealth PA intervention for improving depressive symptoms and increasing PA among postpartum individuals at high PPD risk. Results have implications for the design and delivery of behavioral interventions among vulnerable patients. ClinicalTrials.gov NCT04414696; https://clinicaltrials.gov/ct2/show/NCT04414696. DERR1-10.2196/56882.