Background: Opioid substitution therapy and needle exchanges have reduced blood-borne viruses (BBVs) among people who inject drugs (PWID). Some PWID continue to share injecting equipment. Objectives: To develop an evidence-based psychosocial intervention to reduce BBV risk behaviours and increase transmission knowledge among PWID, and conduct a feasibility trial among PWID comparing the intervention with a control. Design: A pragmatic, two-armed randomised controlled, open feasibility trial. Service users were Steering Group members and co-developed the intervention. Peer educators co-delivered the intervention in London. Setting: NHS or third-sector drug treatment or needle exchanges in Glasgow, London, Wrexham and York, recruiting January and February 2016. Participants: Current PWID, aged ≥ 18 years. Interventions: A remote, web-based computer randomisation system allocated participants to a three-session, manualised, psychosocial, gender-specific group intervention delivered by trained facilitators and BBV transmission information booklet plus treatment as usual (TAU) (intervention), or information booklet plus TAU (control). Main outcome measures: Recruitment, retention and follow-up rates measured feasibility. Feedback questionnaires, focus groups with participants who attended at least one intervention session and facilitators assessed the intervention’s acceptability. Results: A systematic review of what works to reduce BBV risk behaviours among PWID, in-depth interviews with PWID, and stakeholder and expert consultation informed the intervention. Sessions covered improving injecting technique and good vein care, planning for risky situations, and understanding BBV transmission. Fifty-six per cent (99/176) of eligible PWID were randomised: 52 to the intervention group and 47 to the control group. Only 24% (8/34) of male and 11% (2/18) of female participants attended all three intervention sessions. Overall, 50% (17/34) of men and 33% (6/18) of women randomised to the intervention group and 47% (14/30) of men and 53% (9/17) of women randomised to the control group were followed up 1 month post intervention. Variations were reported by location. The intervention was acceptable to both participants and facilitators. At 1 month post intervention, no increase in injecting in ‘risky’ sites (e.g. groin, neck) was reported by participants who attended at least one session. PWID who attended at least one session showed a trend towards greater reduction in injecting risk behaviours, a greater increase in withdrawal planning and were more confident about finding a vein. A mean cost of £58.17 per participant was calculated for those attending one session, £148.54 for those attending two sessions and £270.67 for those attending all three sessions, compared with £0.86 in the control group. Treatment costs across the centres vary as a result of the different levels of attendance, as total session costs are divided by attendees to obtain a cost per attendee. The economic analysis suggests that a cost-effectiveness study would be feasible given the response rates and completeness of data. However, we have identified aspects where the service use questionnaire could be abbreviated given the low numbers reported in several care domains. No adverse events were reported. Conclusions: As only 19% of participants attended all three intervention sessions and 47% were followed up 1 month post intervention, a future definitive randomised controlled trial of the intervention is not feasible. Exposure to information on improving injecting techniques did not encourage riskier injecting practices or injecting frequency, and benefits were reported among attendees. The intervention has the potential to positively influence BBV prevention. Harm reduction services should ensure that the intervention content is routinely delivered to PWID to improve vein care and prevent BBVs.