BACKGROUND: Brief messaging interventions, including Short Message Service (SMS) text-messages, delivered via mobile device platforms, show promise to support and improve treatment adherence. To understand how these interventions work, and to facilitate transparency, we need clear descriptions of the intervention development process. METHOD: We describe and reflect on the process of designing and pretesting an evidence- and theory-informed brief messaging intervention, to improve diabetes treatment adherence in sub-Saharan Africa. We followed the stepwise approach recommended by the Medical Research Council, United Kingdom (MRC UK) Framework for Development and Evaluation of Complex Health Interventions and guidance for mobile health intervention development. RESULTS: We used a four-phase, iterative approach that first generated primary and secondary evidence on the lived experience of diabetes, diabetes treatment services and mobile-phone use. Second, we designed a type 2 diabetes-specific, brief text-message library, building on our previous hypertension text-message library, as well as drawing on the primary and secondary data from phase one, and on expert opinion. We then mapped the brief text-messages onto behaviour change (COM-B) theoretical constructs. Third, we refined and finalised the newly developed brief text-message library through stakeholder consultation and translated it into three local languages. Finally, we piloted the intervention by pre-testing the automated delivery of the brief text-messages in the trial sites in Malawi and South Africa. The final SMS text Adherence suppoRt for people with type 2 diabetes (StAR2D) intervention was tested in a randomised controlled trial in Malawi and South Africa (trial registration: ISRCTN70768808 ). CONCLUSION: The complexity of public health interventions requires that we give more attention to intervention development work. Our documentation and reflection on the StAR2D intervention development process promotes transparency, replicability, assessment of intervention quality, and comparison with other studies.