BackgroundIn South Africa, the prevalence of gestational diabetes (GDM) is growing, concomitant with the dramatically increasing prevalence of overweight/obesity among women. There is an urgent need to develop tailored interventions to support women with GDM to mitigate pregnancy risks and to prevent progression to type 2 diabetes post-partum. The IINDIAGO study aims to develop and evaluate an intervention for disadvantaged GDM women attending three large, public-sector hospitals for antenatal care in Cape Town and Soweto, SA. This paper offers a detailed description of the development of a theory-based behaviour change intervention, prior to its preliminary testing for feasibility and efficacy in the health system.MethodsThe Behaviour Change Wheel (BCW) and the COM-B model of behaviour change were used to guide the development of the IINDIAGO intervention. This framework provides a systematic, step-by-step process, starting with a behavioural analysis of the problem and making a diagnosis of what needs to change, and then linking this to intervention functions and behaviour change techniques to bring about the desired result. Findings from primary formative research with women with GDM and healthcare providers were a key source of information for this process.ResultsKey objectives of our planned intervention were 1) to address women’s evident need for information and psychosocial support by positioning peer counsellors and a diabetes nurse in the GDM antenatal clinic, and 2) to offer accessible and convenient post-partum screening and counselling for sustained behaviour change among women with GDM by integrating follow-up into the routine immunisation programme at the Well Baby clinic. The peer counsellors and the diabetes nurse were trained in patient-centred, motivational counselling methods.ConclusionsThis paper offers a rich description and analysis of designing a complex intervention tailored to the challenging contexts of urban South Africa. The BCW was a valuable tool to use in designing our intervention and tailoring its content and format to our target population and local setting. It provided a robust and transparent theoretical foundation on which to develop our intervention, assisted us in making the hypothesised pathways for behaviour change explicit and enabled us to describe the intervention in standardised, precisely defined terms. Using such tools can contribute to improving rigour in the design of behavioural change interventions.Trial registrationFirst registered on 20/04/2018, Pan African Clinical Trials Registry (PACTR): PACTR201805003336174.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12889-023-15586-y.