Background: People who inject drugs (PWID) are at greater risk of developing bacterial skin and soft tissue infections (SSTI) than the general population. UK prevention interventions have achieved limited impact on the rising prevalence of SSTI among PWID. Innovative harm reduction interventions are needed. We present our approach to the co-development of a personalised, behavioural intervention, REACT (REducing bACTerial infections), which aims to prevent bacterial SSTI among PWID.Methods: We followed the interrelated steps of the Person-Based Approach for intervention planning and development: (i) collating evidence, including published literature and consultations with PWID (n=15), service providers (n=6), and stakeholders (n=11); (ii) developing guiding principles; (iii) undertaking a behavioural analysis; (iv) developing a logic model, and; (v) designing and refining intervention materials. Results: Published literature highlighted structural barriers to safer injecting practices, such as access to hygienic injecting environments, homelessness and social exclusion. Practices associated with bacterial SSTI included: (i) handwashing / injection-site swabbing; (ii) overuse of acidifier; (iii) use of non-sterile water for injection preparation; (iv) reuse of injecting equipment; and; (v) lack of injecting site rotation. Consultations indicated vein care and minimisation of pain as priorities, while emphasising the importance of service provider-client relationships during intervention delivery. The need to deliver REACT in a non-judgemental and non-stigmatising manner, and to address stigma among PWID when communicating intervention messages, were additional priorities. Providing practical, tailored resources was identified as important to address environmental constraints to safer injecting practices. Findings were used to iteratively refine the REACT intervention. Conclusion: Our evidence-based, collaborative and iterative approach, enabled alignment of the aim of the behavioural intervention to priorities of PWID, ensuring an appealing and acceptable intervention design while maximising likely feasibility of delivery and behaviour change. Piloting will establish the feasibility and acceptability of REACT to service providers and PWID.