Temperatures in Southern Africa are rising at twice the global rate, with major health implications for maternal and newborn health. The HAPI study aims to advance heat-adaptation policies and practices in low- and middle-income countries. We will develop and test a multi-level (from individual- to international policy-level) and multi-component intervention, encompassing behavioural, built environment, nature-based, health services, and policy components. Following multi-layered ethnographic observations, and thermal, emissions and cost-consequences modelling, we will co-produce an intervention package based on progressively-optimised programme theory. The intervention will be refined over two action-research cycles, each lasting six months. We will apply a quasi-experimental design involving 1600 women pre- and post-intervention to assess process, feasibility, cost and biomedical outcomes. Activities will take place in six maternity facilities, and surrounding communities and households in urban Tshwane, South Africa, and rural Mount Darwin District, Zimbabwe. These areas have contrasting climatic conditions, economies, and socio-cultural practices, allowing for greater transferability of findings. The study builds on a Horizons Europe project and related field experience. Capacity-building at individual-, institutional- and societal-level will be integrated into all research activities, and take place within inclusive and diverse research environments. Key words: Climate change, maternal and newborn health, heat-related adaptation, southern Africa