BACKGROUND: Injecting drug use (IDU) is a growing concern in Tanzania compounded by reports of high-risk injecting and sexual risk behaviours among people who inject drugs (PWID). These behaviours have implications for transmission of blood-borne viruses, including HIV and hepatitis C (HCV). METHODS: We recruited 267 PWID (87% male) from Temeke District, Dar-es-Salaam through snowball and targeted sampling. A behavioural survey was administered alongside repeated rapid HIV and HCV antibody testing. HIV and HCV prevalence estimates with 95% confidence intervals (CIs) were calculated. RESULTS: Among PWID, 34.8% (95%CI 29.1-40.9) tested HIV positive (29.9% of males and 66.7% of females); 27.7% (95%CI 22.0-34.0) tested HCV antibody positive. Almost all (97%) participants were aware of HIV and 34% of HCV. 45% of male and 64% of female PWID reported a previous HIV test; only five (2%) PWID reported a previous HCV test. Of HIV and HCV positive tests, 73% and 99%, respectively, represented newly diagnosed infections. CONCLUSION: High prevalence of HIV and HCV were detected in this population of PWID. Rapid scale-up of targeted primary prevention and testing and treatment services for PWID in Tanzania is needed to prevent further transmission and consequent morbidities.