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.