Background The burden of HIV in transgender women (transwomen) in Brazil remains unknown. We aimed to estimate HIV prevalence among transwomen in Rio de Janeiro and to identify predictors of newly diagnosed HIV infections. Methods We recruited transwomen from Rio de Janeiro, Brazil, by respondent-driven sampling. Eligibility criteria were self-identifi cation as transwomen, being 18 years of age or older, living in Rio de Janeiro or its metropolitan area, and having a valid peer recruitment coupon. We recruited 12 seed participants from social movements and formative focus groups who then used peer recruitment coupons to refer subsequent peers to the study. We categorised participants as HIV negative, known HIV infected, or newly diagnosed as HIV infected. We assessed predictors of newly diagnosed HIV infections by comparing newly diagnosed with HIV-negative participants. We derived population estimates with the Respondent-Driven Sampling II estimator. Findings Between Aug 1, 2015, and Jan 29, 2016, we enrolled 345 eligible transwomen. 29·1% (95% CI 23·2–35·4) of participants had no previous HIV testing (adjusted from 60 participants), 31·2% (18·8–43·6) had HIV infections (adjusted from 141 participants), and 7·0% (0·0–15·9) were newly diagnosed as HIV infected (adjusted from 40 participants). We diagnosed syphilis in 28·9% (18·0–39·8) of participants, rectal chlamydia in 14·6% (5·4–23·8), and gonorrhoea in 13·5% (3·2–23·8). Newly diagnosed HIV infections were associated with black race (odds ratio 22·8 [95% CI 2·9–178·9]; p=0·003), travesti (34·1 [5·8–200·2]; p=0·0001) or transsexual woman (41·3 [6·3–271·2]; p=0·0001) gender identity, history of sex work (30·7 [3·5–267·3]; p=0·002), and history of sniffi ng cocaine (4·4 [1·4–14·1]; p=0·01). Interpretation Our results suggest that transwomen bear the largest burden of HIV among any population at risk in Brazil. The high proportion of HIV diagnosis among young participants points to the need for tailored long-term health-care and prevention services to curb the HIV epidemic and improve the quality of life of transwomen in Brazil.