Abstract Background There is a critical need for an intervention to improve nurses’ eating and physical activity behaviours. As nurses spend a substantial proportion of their waking hours at work, concerted efforts to deliver such interventions in the workplace is growing. This study formed part of a multiphase programme of research that aimed to systematically develop an evidence-based and theory-informed workplace intervention to promote changes in eating and physical activity among nurses. Methods The intervention was developed iteratively, in line with Medical Research Council complex intervention guidelines. It involved four activities: (1) identifying the evidence base, (2) understanding the determinants of nurses’ eating and physical activity behaviour change through theory-based qualitative interviews and survey, (3) identifying intervention options using the Behaviour Change Wheel, and (4) specifying intervention content and implementation options using a taxonomy of behaviour change techniques. Results Data from 13 randomised controlled trials indicated that workplace-based behaviour change interventions targeted to this population are effective in changing behaviour. The evidence base was, however, limited in quantity and quality. Nurses’ beliefs about important factors determining their eating and physical activity behaviour were identified across 16 qualitative interviews and 245 survey responses, and key determinants included environmental context and resources, behavioural regulation, emotion, beliefs about consequences, knowledge and optimism. Based on these findings, 22 behaviour change techniques suitable for targeting the identified determinants were identified and combined into a potential workplace intervention. Conclusions An evidence-based and theory-informed intervention tailored to the target population and setting has been explicitly conceptualised using a systematic approach. The proposed intervention addresses previous evidence gaps for the user population of nurses. Further to this, such an intervention, if implemented, has the potential to impact nurses’ eating and physical activity behaviours and in turn, the health of nurses and the quality of healthcare delivery.