Obesity is one of the highest metabolic risk factors associated with morbidity and mortality in the UK, with two-thirds of adults in the UK classed as overweight or obese. Whole systems approaches can be effective in tackling this public health challenge through stakeholder and key partner engagement. This article describes the co-design process for a social marketing obesity prevention campaign taking a whole systems approach based on the COM-B model of behaviour change. Development of social marketing campaigns through Agencies is often hampered by rapid turnaround and short timescales; we highlight how drawing on existing knowledge and co-design with beneficiaries can support the design, delivery and implementation of a social marketing behaviour change campaign.