BACKGROUND: Social media technologies offer new approaches to HIV prevention and promotion of testing. We examined the efficacy of the Harnessing Online Peer Education (HOPE) social media intervention to increase HIV testing among men who have sex with men (MSM) in Peru. METHODS: In this cluster randomised controlled trial, Peruvian MSM from Greater Lima (including Callao) who had sex with a man in the past 12 months, were 18 years of age or older, were HIV negative or serostatus unknown, and had a Facebook account or were willing to create one (N=556) were randomly assigned (1:1) by concealed allocation to join intervention or control groups on Facebook for 12 weeks. For the intervention, Peruvian MSM were trained and assigned to be HIV prevention mentors (peer-leaders) to participants in Facebook groups. The interventions period lasted 12 weeks. Participants in control groups received an enhanced standard of care, including standard offline HIV prevention available in Peru and participation in Facebook groups (without peer leaders) that provided study updates and HIV testing information. After accepting a request to join the groups, continued participation was voluntary. Participants also completed questionnaires on HIV risk behaviours and social media use at baseline and 12 week follow-up. The primary outcome was the number of participants who received a free HIV test at a local community clinic. The facebook groups were analysed as clusters to account for intracluster correlations. This trial is registered with ClinicalTrials.gov, number NCT01701206. FINDINGS: Of 49 peer-leaders recruited, 34 completed training and were assigned at random to the intervention Facebook groups. Between March 19, 2012, and June 11, 2012, and Sept 26, 2012, and Dec 19, 2012, 556 participants were randomly assigned to intervention groups (N=278) or control groups (N=278); we analyse data for 252 and 246. 43 participants (17%) in the intervention group and 16 (7%) in the control groups got tested for HIV (adjusted odds ratio 2·61, 95% CI 1·55–4·38). No adverse events were reported. INTERPRETATION: Development of peer-mentored social media communities seemed to be an efficacious method to increase HIV testing among high-risk populations in Peru. Results suggest that the HOPE social media intervention could improve HIV testing rates among MSM in Peru. FUNDING: National Institute of Mental Health.