BACKGROUND: There are few data available regarding the use of on-demand pre-exposure prophylaxis (PrEP) for HIV prevention. We aimed to assess PrEP effectiveness, adherence, and safety in adults using daily or on-demand PrEP. METHODS: We conducted a prospective observational cohort study (ANRS PREVENIR) at 26 sites in the Paris region, France. We enrolled HIV-negative adults (aged ≥18 years) at high risk of HIV infection who were starting or continuing PrEP. PrEP was prescribed as a fixed-dose combination of tenofovir disoproxil and emtricitabine (245 mg and 200 mg, respectively, per pill). PrEP could be prescribed as a daily regimen with one pill per day or, in men who have sex with men (MSM) or in transgender women who have sex with men, as an on-demand regimen following the IPERGAY dosing recommendation. At enrolment and every 3 months thereafter, participants were tested for HIV and provided information regarding the PrEP dosing regimen used. Adherence to PrEP was assessed by self-report and by tenofovir diphosphate concentrations in dried blood spots. The primary outcome of HIV-1 incidence was assessed using Poisson regression among participants who started PrEP. This study is registered with ClinicalTrials.gov, NCT03113123, and EudraCT, 2016A0157744. FINDINGS: Between May 3, 2017, and May 2, 2019, 3082 people were assessed for eligibility and 3065 participants were enrolled. 3056 (99·7%) of 3065 participants reported using PrEP and were included in the analyses. The median age was 36 years (IQR 29-43), 1344 (44·0%) of 3056 participants were PrEP-naive, and 3016 (98·7%) were MSM. At enrolment, 1540 (50·5%) of 3049 participants opted for daily PrEP dosing and 1509 (49·5%) opted for on-demand PrEP dosing; these proportions remained stable during follow-up. Median follow-up was 22·1 months (IQR 15·9-29·7) and incidence of study discontinuation was 17·6 participants (95% CI 16·5-18·7) per 100 person-years. At the data cutoff on Sept 30, 2020, there had been six HIV-1 seroconversions (three participants using daily PrEP and three using on-demand PrEP; all were MSM) over 5623 person-years. Overall HIV-1 incidence was 1·1 cases (95% CI 0·4-2·3) per 1000 person-years, and did not differ between participants using daily PrEP and those using on-demand PrEP (incidence rate ratio 1·00, 95% CI 0·13-7·49; p=0·99). Four participants (two using daily PrEP and two using on-demand PrEP) discontinued PrEP due to treatment-related adverse events (nausea [n=2], vomiting and diarrhoea [n=1], and lumbar pain [n=1]). INTERPRETATION: In this study, which enrolled mainly MSM, HIV-1 incidence on PrEP was low and did not differ between participants using daily PrEP and those using on-demand PrEP. On-demand PrEP therefore represents a valid alternative to daily PrEP for MSM, providing greater choice in HIV prevention. FUNDING: ANRS/Maladies Infectieuses Emergentes, Gilead Sciences, SIDACTION, and Région Ile de France. TRANSLATION: For the French translation of the abstract see Supplementary Materials section.