BACKGROUND:HCV transmission remains high in prisons globally. Understanding injecting risk behaviours in prisons is crucial to effectively develop and implement HCV prevention programs in this setting including treatment as prevention. METHODS:HITS-p is a cohort study which enrolled people with a history of injecting drug use in prisons in NSW, Australia from 2005 to 2013. Participants completed an interview at enrolment and follow-up visits to determine injecting behaviours. Generalized estimating equation (GEE) and logistic regression methods were used to assess injecting risk behaviours prior to and following prison entry and to investigate injecting risk behaviours in prison. RESULTS:Overall, 499 participants with a history of injecting drug use were included (median age, 26 years; 65% male). Participants were significantly less likely to inject drugs following incarceration. Among injectors, participants were less likely to inject ≥weekly but more likely to share a needle/syringe. At enrolment, the proportion reporting any injecting, ≥weekly injecting, and needle/syringe sharing in prison was highest among younger individuals. Younger age was associated with both re-initiation and continuation of injecting drug use following prison entry. Among those continuously imprisoned, younger age was associated with increased odds of any injecting, ≥weekly injecting, and sharing a needle/syringe. CONCLUSIONS:Upon entry to prison, injecting drug use decreased but syringe sharing increased among injectors. Younger individuals are most likely to exhibit high-risk injecting behaviours in prison. These data highlight the need for improved HCV prevention strategies (including improved needle/syringe access and scale up of HCV therapy) for those at increased risk of HCV transmission in prison, including younger individuals.