BACKGROUND: Strengthening engagement of female sex workers with health services is needed to eliminate HIV. We assessed the efficacy of a targeted combination intervention for female sex workers in Zimbabwe. METHODS: We did a cluster-randomised trial from 2014 to 2016. Clusters were areas surrounding female sex worker clinics and were enrolled in matched pairs. Sites were randomly assigned (1:1) to receive usual care (free sexual-health services supported by peer educators, including HIV testing on demand, referral for antiretroviral therapy [ART], and health education) or an intervention that supported additional regular HIV testing, on-site initiation of ART, pre-exposure prophylaxis, adherence, and intensified community mobilisation. The primary outcome was the proportion of all female sex workers with HIV viral load 1000 copies per mL or greater, assessed through respondent-driven sampling surveys. We used an adapted cluster-summary approach to estimate risk differences. This trial is registered with Pan African Clinical Trials Registry, number PACTR201312000722390. RESULTS: We randomly assigned 14 clusters to usual care or the intervention (seven in each group). 3612 female sex workers attended clinics in the usual-care clusters and 4619 in the intervention clusters during the study. Half as many were tested (1151 vs 2606) and diagnosed as being HIV positive (546 vs 1052) in the usual-care clusters. The proportion of all female sex workers with viral loads of 1000 copies per mL or greater fell in both study groups (from 421 [30%] of 1363 to 279 [19%] of 1443 in the usual-care group and from 399 [30%] of 1303 to 240 [16%] of 1439 in the intervention group), but with a risk difference at the end of the assessment period of only -2·8% (95% CI -8·1 to 2·5, p=0·23). Among HIV-positive women, the proportions with viral loads less than 1000 copies per mL were 590 (68%) of 869 in the usual-care group and 588 (72%) of 828 in the intervention group at the end of the assessment period, adjusted risk difference of 5·3% (95% CI -4·0 to 14·6, p=0·20). There were no adverse events. INTERPRETATION: Our intervention of a dedicated programme for female sex workers led to high levels of HIV diagnosis and treatment. Further research is needed to optimise programme content and intensity for the broader population. FUNDING: UN Population Fund (through Zimbabwe's Integrated Support Fund funded by UK Department for International Development, Irish Aid, and Swedish International Development Cooperation Agency).