Background: Perinatal depression and anxiety are significant public health concerns, affecting up to 1 in 5 women globally, with disproportionate burden carried by women in regional, rural, and remote communities where structural and social inequities amplify vulnerability. Access to perinatal mental health support in these settings is severely constrained by geographical isolation, workforce shortages, financial barriers, and a lack of culturally safe services. Prevention is recognized as critical to reducing this burden, with evidence suggesting that effective preventive approaches can reduce population-level illness by up to 40% and alleviate downstream demand on overstretched services. Digital mental health interventions hold promise for improving access to support, yet few are co-designed with underserved perinatal populations. Objective: This study aimed to identify the mental health needs of perinatal women in regional, rural, and remote communities and to co-design a framework for a preventive social media–based intervention informed by platform-specific affordances and constraints, using Northern Queensland, Australia, as an exemplar. Methods: Using a participatory co-design approach, 26 perinatal women (21 postnatal and 5 antenatal) and 8 mental health care professionals from regional, rural, and remote Northern Queensland participated in focus groups or interviews, supplemented by ongoing consultation with a community advisory group comprising lived experience representatives, clinicians, and local community leaders. Qualitative data were analyzed using reflexive thematic analysis to identify core community mental health needs. Identified needs were then examined through a needs-affordances framework to determine how specific platform features could address, enable, or constrain those needs in the context of a preventive intervention. Results: Five core mental health needs were identified: (1) social connection and support; (2) personalized and respectful health care; (3) information that empowers; (4) place-based and culturally safe support; and (5) accessible, low-burden digital formats. Participants viewed social media as a potentially useful platform for fostering peer connection, normalizing perinatal experiences, and providing timely psychoeducation. However, both mothers and professionals expressed concerns about misinformation, harmful social comparison, and privacy risks that must be proactively addressed in program design. These insights were synthesized into a set of prototype design guidelines specifying recommended content, features, tone, and delivery formats to inform subsequent intervention development. Conclusions: This study provides a place-based, co-designed needs-affordances framework to guide the development of a preventive social media–based intervention for perinatal mental health support in regional, rural, and remote communities. The findings demonstrate that social media is an acceptable and promising platform for preventive perinatal mental health support in these settings, provided that design is driven by community need, platform affordances are systematically analyzed, and known risks are explicitly mitigated. These findings address a significant gap in the literature and offer a replicable methodological approach for co-designing contextually relevant digital mental health interventions with underserved populations.