Background: New parents face increased risks of emotional distress and relationship dissatisfaction. Digital interventions increase support access, but few preventive programs are optimized for both parents. Objective: This study aims to conduct the first randomized controlled trial on universal self-guided digital programs to support positive perinatal adjustment of both mothers and fathers. Effects of childcare information (Baby Care) and information plus an interactive program (Baby Steps Wellbeing) were compared from the third trimester baseline to 3 and 6 months subsequently. Methods: The study recruited 388 co-parenting male-female adult couples expecting their first single child (26-38 weeks' gestation), using web-based registration. Most (337/388, 86.8%) were obtained from prenatal hospital classes. Couples' randomization was automated and stratified by Edinburgh Postnatal Depression Scale (EPDS) scores (50% couples scored high if either mother >7, father >5). All assessments were web-based self-reports: the EPDS and psychosocial quality of life were primary outcomes; relationship satisfaction, social support, and self-efficacy for parenting and support provision were secondary. Linear mixed models provided intention-to-treat analyses, with linear and quadratic effects for time and random intercepts for participants and couples. Results: Selection criteria were met by 63.9% (248/388) of couples, who were all randomized. Most participants were married (400/496, 80.6%), tertiary educated (324/496, 65.3%), employed full time (407/496, 82%), and born in Australia (337/496, 67.9%). Their mean age was 32.2 years, and average gestation was 30.8 weeks. Using an EPDS cutoff score of 13, 6.9% (18/248) of men, and 16.1% (40/248) of women screened positive for depression at some time during the 6 months. Retention of both partners was 80.6% (201/248) at the 6-month assessments, and satisfaction with both programs was strong (92% ≥50). Only 37.3% (185/496) of participants accessed their program more than once, with higher rates for mothers (133/248, 53.6%) than fathers (52/248, 20.9%; P<.001). The EPDS, quality of life, and social support did not show differential improvements between programs, but Baby Steps Wellbeing gave a greater linear increase in self-efficacy for support provision (P=.01; Cohen d=0.26) and lower reduction in relationship satisfaction (P=.03; Cohen d=0.20) than Baby Care alone. Mothers had greater linear benefits in parenting self-efficacy over time than fathers after receiving Baby Steps Wellbeing rather than Baby Care (P=.01; Cohen d=0.51). However, the inclusion of program type in analyses on parenting self-efficacy and relationship satisfaction did not improve model fit above analyses with only parent gender and time. Conclusions: Three secondary outcomes showed differential benefits from Baby Steps Wellbeing, but for one (parenting self-efficacy), the effect only occurred for mothers, perhaps reflecting their greater program use. Increased engagement will be needed for more definitive testing of the potential benefits of Baby StepsWellbeing for perinatal adjustment.