BACKGROUND: To achieve reductions in HIV incidence, we need strategies to engage key population at risk for HIV in low-income and middle-income countries. We evaluated the effectiveness of integrated care centres in India that provided single-venue HIV testing, prevention, and treatment services for people who inject drugs (PWID) and men who have sex with men (MSM). METHODS: We did baseline respondent-driven sampling surveys in 27 sites across India, and selected 22 of these (12 PWID and ten MSM) for a cluster randomised trial on the basis of high HIV prevalence and logistical considerations. We used stratified (by PWID and MSM), restricted randomisation to allocate sites to either the integrated care intervention or usual care (11 sites per group). We implemented integrated care centres in 11 cities (six PWID integrated care centres embedded within opioid agonist treatment centres and five MSM centres within government-sponsored health services), with a single integrated care centre per city in all but one city. After a 2-year intervention phase, we did respondent-driven sampling evaluation surveys of target population members who were aged 18 years or older at all sites. The primary outcome was self-reported HIV testing in the previous 12 months (recent testing), determined via the evaluation survey. We used a biometric identification system to estimate integrated care centre exposure (visited an integrated care centre at least once) among evaluation survey participants at intervention sites. This trial is registered with ClinicalTrials.gov, number NCT01686750. FINDINGS: Between Oct 1, 2012, and Dec 19, 2013, we recruited 11 993 PWID and 9997 MSM in the baseline survey and, between Aug, 1 2016, and May 27, 2017, surveyed 11 721 PWID and 10 005 MSM in the evaluation survey using respondent-driven sampling, across the 22 trial sites. During the intervention phase, integrated care centres provided HIV testing for 14 698 unique clients (7630 PWID and 7068 MSM. In the primary population-level analysis, recent HIV testing was 31% higher at integrated care centres than at usual care sites (adjusted prevalence ratio [PR] 1·31, 95% CI 0·95-1·81, p=0·09). Among survey participants at intervention sites, integrated care centre exposure was lower than expected (median exposure 40% at PWID sites and 24% at MSM sites). In intervention sites, survey participants who visited an integrated care centre were more likely to report recent HIV testing than were participants who had not (adjusted PR 3·46, 2·94-4·06). INTERPRETATION: Although integrated care centres increased HIV testing among visitors, our low exposure findings suggest that the scale-up of a single integrated care centre in most cities was insufficient to serve the large PWID and MSM populations. Future work should address the use of population size estimates to guide the dose of combination HIV interventions targeting key populations. FUNDING: US National Institutes of Health and the Elton John AIDS Foundation.