Full text article available at https://doi.org/10.1371/journal.pone.0249061Blood transfusion saves many people every year that would otherwise have died. The present study aimed to provide an update and insightful information regarding prevalence of the common Transfusion-Transmitted Infections (TTIs) and associated factors among blood donors in Tanzania. This was a cross-sectional study involving retrospectively collected data of blood donors from the Tanzania Northern Zone Blood Transfusion Center between 2017 and 2019. Descriptive statistics were performed to describe characteristics of the blood donors. Univariable and multivariable logistic regression analyses were performed to determine association between prevalence of TTIs and socio-demographic factors. P-value<0.05 was considered statistically significant. A total of 101,616 blood donors were included in the present study of which 85,053(83.7%) were males while16,563 (16.3%) were females. Of all participants, the majority 45,400(44.7%) were aged between 18 and 25 years; 79,582 (78.3%) were voluntary non-remunerated donors while 22,034 (21.7%) were replacement donors. The vast majority of them 99,626 (98%) were first time blood donors while 1990 (2%) were multiple donors. The overall prevalence of TTIs was 10.1% (10,226 out of 101,616) of which the leading was HBV accounting for 5.1% (5,264 out of 101,616). Being a replacement donor was associated with all the four types of TTIs: HIV (AOR=1.22,95% CI= 1.10–1.35), HBV (AOR= 1.35,95% CI= 1.27–1.44),HCV (AOR= 1.28,95% CI= 1.12–1.46), and syphilis (AOR= 1.33,95% CI=1.20–1.48). Our study has demonstrated that Tanzania has relatively high prevalence of TTIs compared to some countries in Sub-Saharan Africa. HBV infection seems to be the most common infection among blood donors and replacement blood donors are at a higher risk of harboring the commonest TTIs among blood donors