["Applied Psychology: Health and Well-Being, Volume 18, Issue 4, August 2026. ", "\nAbstract\nPolice work can be stressful, negatively affecting well‐being and familial relationships. But, the influence of family‐based interventions on such outcomes is unknown. Using co‐creation, we employed the ‘Double Diamond’ framework in four British police forces to develop, deliver and evaluate ‘1TeamActive’ – a physical activity intervention for police workers and their families. Multiple stakeholders (Team Police UK, Sport England, four UK police forces, police workers and families) participated in four longitudinal research phases. In Phase 1, literature reviews ‘Discovered’ the stress impacts of policing (workers and families) and explored physical activity as a potential mitigator. In Phase 2, stakeholders ‘Defined’ an initial intervention plan, which was ‘Developed’ in Phase 3 to produce context‐specific delivery. In Phase 4, a 12‐week instructor‐led physical activity intervention was ‘Delivered’ and evaluated. One hundred and two police workers and their families completed the intervention. Repeated Measures Multivariate Analysis of Variance (RM‐MANOVA) indicated that physical activity, well‐being and work‐related variables improved significantly from pre‐ to post‐intervention, Pillai's trace = .65, F(8, 59) = 13.71, p < .001. Evaluation interviews indicated participants' perceived positive individual and familial benefits from completing ‘1TeamActive’. Our findings provide novelty and rigour with respect to intervention development, and the feasibility and acceptability findings support the intervention's potential to improve selected outcomes. Controlled trials are needed to determine efficacy.\n"]