||Introduction and Aims: The aims of this study were to describe the prevalences of injection practices and needle/syringe sharing in people who use drugs in French prisons, and to investigate associated factors. Design and Methods: Using the ANRS-Coquelicot survey (2011−2013), a random sample of 1718 people who used drugs in free society was included. Information regarding a history of incarceration, drug-injection practices inside prison and needle/syringe sharing was collected during interviews. Results: In our sample, 65.5% reported a history of injection and 57.4% had been incarcerated at least once. Among those who reported both of these conditions, 14% reported injection practices inside prison, 40.5% of whom had shared needles/syringes. In the multivariable model, the following variables were associated with injection practices inside prison: being a Russian-speaking detainee, having spent more time in prison, and having started to inject before 1996 and especially before 1987. Being Russian speaking was also associated with needle/syringe sharing in prison. Discussion and Conclusions: The prevalences of injection practices and needle/syringe sharing in prisons are alarmingly high. Effective interventions to prevent the transmission of infectious diseases among people who use drugs in the prison setting are essential. The implementation of international recommendations on the principle of equivalence between prisons and the community is still very limited in most countries, and should be complemented with tailored interventions for the most vulnerable prison populations, especially Russian-speaking detainees.