To eliminate mother-to-child transmission of HIV (EMTCT), scalable strategies to enhance antiretroviral adherence for both antiretroviral therapy (ART) and pre-exposure prophylaxis (PrEP) are needed as part of integrated HIV and maternal-child health services. We developed Tonse Pamodzi ("all of us together"), an adaptable intervention integrating biomedical and behavioral components to support HIV treatment and prevention. We describe our intervention development process, which comprised formative qualitative research, a review of the literature, and technical input from stakeholders representing the community, health systems, and policymakers. The resulting intervention, described herein, integrates patient-centered counseling and engagement of a patient-selected adherence supporter for pregnant and breastfeeding women initiating ART or PrEP. Patients receiving the intervention engage in Integrated Next Step Counseling (iNSC) sessions delivered by trained counselors to build and maintain adherence skills. Each patient also has the option of selecting an adherence supporter (partner, family member, or friend) who may participate in iNSC sessions and provide adherence support outside of these sessions. This flexible intervention is adaptable not only to ART or PrEP use, but also to the needs and preferences of each woman and the clinical context. If shown to be acceptable and feasible, the Tonse Pamodzi intervention may be an important tool in continuing efforts for EMTCT.