Background Mothers who identify as Black or African American are more likely to report depressed moods in late pregnancy and early postpartum and have the lowest rates of human milk feeding compared with all other racial groups in the United States. Internet interventions offer the potential to extend preventative and supportive services as they address key barriers, particularly for those navigating the complex and vulnerable early postpartum period. However, there is limited evidence on the feasibility of such interventions for preventing perinatal mental health disorders and improving human milk feeding outcomes in Black mothers. Objective This pilot study aimed to assess the feasibility and preliminary findings of a web-based cognitive behavioral therapy–based internet intervention, with and without human milk feeding education and support, to prevent perinatal depression and promote human milk feeding in Black mothers. Methods Participants were Black-identifying individuals between 20 and 28 weeks of pregnancy with human milk feeding intention and mild to moderate depressive symptoms (Patient Health Questionnaire scores 5-14). Participants were randomized to either Sunnyside, a 6-week cognitive behavioral therapy–based web-based intervention, or Sunnyside Plus, which included additional education and support to promote human milk feeding. Assessments occurred at baseline, third trimester (end of antenatal treatment), 6 weeks postpartum (end of postpartum treatment), and 12 weeks postpartum. The primary focus of this randomized pilot trial was the feasibility and preliminary outcomes of mental health and human milk feeding. Results A total of 22 tertiary-educated participants were randomized. The mean number of log-ins was 7.3 (SD 5.3) for Sunnyside and 13.8 (SD 10.5) for Sunnyside Plus. Scores of depression and anxiety measures remained below the clinical threshold for referral to treatment in both groups. All the participants initiated human milk feeding (18/18, 100%). Most participants reported at least some human milk feeding at both 6 and 12 weeks postpartum (6/7, 86%; 11/11, 100%, or 10/10, 100%, for Sunnyside and Sunnyside Plus, respectively). Conclusions The results suggest that tertiary-educated Black mothers at risk for perinatal depression and who intended to human milk feed were receptive to and satisfied with a web-based cognitive behavioral therapy–based internet intervention, with and without human milk feeding education and support. Preliminary findings indicate that both Sunnyside and Sunnyside Plus interventions have the potential to affect symptoms of depression, anxiety, and human milk feeding outcomes. Trial Registration ClinicalTrials.gov NCT04128202; https://www.clinicaltrials.gov/ct2/show/NCT04128202