Hepatitis B (HBV) is a major public health threat in Sierra Leone. Pregnant women are disproportionately impacted, yet little is known about the epidemiology of HBV in this group. We conducted a cross-sectional study of pregnant women aged ≥16 years receiving antenatal care across 10 community health centers in Freetown from July to September 2021 to assess the prevalence and associated factors of HBsAg seropositivity. A logistic regression was used to identify the predictors of HBsAg seropositivity. In total, 394 pregnant women were screened. The mean age was 24.4 ± 4.9 years, 78.2% were married, and 47.2% were in the second trimester. Only 1% had received the HBV vaccine. The prevalence of HBsAg was 7.9%, while HIV was 5.8% and HIV/HBV co-infection was 0.3%. Regarding high-risk practices, 76.6% reported female genital circumcision, 41.9% ear piercing, 29.0% endorsed multiple sexual partners, and 23.6% reported sexually transmitted infections. In the logistic regression analysis, having a husband/partner with HBV (adjusted odds ratio (aOR): 6.54; 95% CI: [1.72–24.86]; p = 0.006) and residing in Central Freetown (aOR: 4.00; 95% CI: [1.46–11.00]; p = 0.007) were independently associated with HBsAg seropositivity. Our findings support the scaling up of HBV services to target pregnant women and their partners for screening and vaccination to help reduce mother-to-child transmission rates in Sierra Leone.