In developing countries there is no routine screening of hepatitis B virus (HBV) infection among pregnant women resulting into limited data on its magnitude. The objective of this study was to determine the prevalence and risk factors associated with active HBV infection among pregnant women attending antenatal clinic (ANC) in Mwanza City, Tanzania. A total of 211 pregnant women were serially enrolled between May and July 2014. Hepatitis B surface antigen (HBsAg) was determined using a rapid Immuno-chromatographic assay. The median age of the study population was 23 years (IQR 20-29 years). Of 211 pregnant women, 61.6% (130/211) were multigravidae. Eight (3.8%) of the pregnant women were positive with HBsAg. There was a significant difference in prevalence between primigravidae and multigravidae (0.8% vs. 8.6%, p=0.017). Active hepatitis B infection among pregnant women in Mwanza city is low and associated with multigravidity. Despite low prevalence of acute hepatitis B infection routine screening of HBsAg and anti HBsAg antibodies, coupled with the vaccination of those at risk should be introduced to prevent hepatitis B infection complications.