Approximately 4% to 8% of pregnant individuals meet the criteria for current posttraumatic stress disorder (PTSD), a known risk factor for a multitude of adverse maternal and child health outcomes. However, PTSD is rarely detected or treated in obstetric settings. Moreover, available prenatal PTSD treatments require in-person services that are often inaccessible due to barriers to care. Thus, web-based interventions offer great potential in extending PTSD treatment to high-risk pregnant individuals by providing affordable, accessible care. However, there are currently no web-based interventions designed specifically for the treatment of PTSD symptoms during pregnancy. This study aims to develop and pilot a 6-week, web-based, cognitive behavioral therapy intervention for PTSD, SunnysideFlex, in a sample of 10 pregnant women with current probable PTSD. Consistent with established guidelines for developing and testing novel interventions, the focus of this pilot study was to evaluate the initial feasibility and acceptability of the SunnysideFlex intervention and preintervention to postintervention changes in PTSD and depression symptoms. This approach will allow for early refinement and optimization of the SunnysideFlex intervention to increase the odds of success in a larger-scale clinical trial. The SunnysideFlex intervention adapted an existing web-based platform for postpartum depression, Sunnyside for Moms, to include revised, trauma-focused content. A total of 10 pregnant women in weeks 16 to 28 of their pregnancy who reported lifetime interpersonal trauma exposure (ie, sexual or physical assault) and with current probable PTSD (scores ≥33 per the PTSD checklist for DSM-5) were enrolled in the SunnysideFlex intervention. Assessments took place at baseline and 6 weeks (postintervention). All participants were retained through the postintervention assessment period. Engagement was high; participants on average accessed 90% of their lessons, logged on to the platform at least weekly, and reported a generally positive user experience. Moreover, 80% (8/10) of participants demonstrated clinically meaningful reductions in PTSD symptoms from baseline to postintervention, and 50% (5/10) of participants no longer screened positive for probable PTSD at postintervention. Most (6/10, 60%) of the participants maintained subclinical depression symptoms from baseline to postintervention. Findings from this small pilot study indicate that SunnysideFlex may be a feasible and acceptable mechanism for delivering PTSD intervention to high-risk, trauma-exposed pregnant women who might otherwise not have opportunities for services. Larger-scale trials of the intervention are necessary to better understand the impact of SunnysideFlex on PTSD symptoms during pregnancy and the postpartum period.