Background <p>Professional support is an important strategy to improve the retention of rural doctors. However, it is poorly defined and understood within the context of rural medicine making it challenging to design and implement. This research aimed to explore what professional support entails and how it works and in what context, to improve whole of rural medical workforce retention.</p> Methods <p>An online expression of interest was circulated to 143 organisations/key informants potentially involved in professional support activities to which 87 replied. Thirty-three of the longest running and comprehensive professional support activities spread across different medical specialties and career stages, locations and target cohorts were selected. Respondents participated in up to two 1-h semi-structured interviews. Realist evaluation involved drawing out patterns about how professional support worked in what context to drive retention of rural doctors, to develop theory. The theory was developed, refined and confirmed with insights from an internal reference and external project advisory group.</p> Results <p>A whole of medical workforce conceptualisation of professional support was defined identifying that in the context of rural medicine, three categories of professional support could drive longer term retention of rural doctors in rural work. These were lifelong career support, sustainable practise support and healthcare and social support. Together, these categories are likely to co-stimulate generative mechanisms (rural doctors’ sense of comfort, confidence, competence, belonging and bonding) to promote shorter term outcomes of feeling valued and connected on the path to longer term retention. Professional support interventions could be scaled up or down over time and should be responsive to the breadth and complexity of work of rural doctors, their level of isolation/autonomy and their socio-cultural stresses.</p> Conclusions <p>The findings articulate the concept of professional support across the rural medical workforce. The results suggest that professional support could be bundled and scaled up or down to address the holistic needs of individual doctors to generate better retention. The theory clarifies a range of professional support activity which could be coordinated through health services, education providers, government and wider agencies to more systematically ensure rural medicine is sustainable.</p>