Fathers' perinatal mental health is a major public health issue, yet few interventions have been developed targeting this group. Fathers face many barriers in accessing perinatal mental health support, including stigma around caregiving and mental health, and thus require careful consideration when designing interventions. This study aimed to examine the feasibility, acceptability, and usability of a mobile app-based intervention for paternal perinatal depression, anxiety, and stress. Following a design science approach, five meta design principles and 15 specific principles were created to guide the intervention design, and a prototype app titled Rover was created. The prototype was evaluated by 43 fathers and 10 mental health clinicians. Participants in both groups rated the app highly for its functionality, clinical content, aesthetics, and digital therapeutic alliance. Qualitative feedback indicated that fathers held particularly favourable views regarding the mood tracking, mindfulness, and goal tracking features. Both groups expressed a preference for more support for the personalisation of content, including more dynamic interactions with the chatbot support feature. To our knowledge, this is the first app-based mental health intervention designed specifically for fathers, with study results providing guidance to the field on developing digital health initiatives for this population.