Background: Human immunodeficiency virus (HIV) and hepatitis virus (HBV) coinfection are common due to shared modes of transmission between these viruses. Also studies have shown that HIV appears to be a risk factor for reactivation of hepatitis B in patients who have developed hepatitis B surface antibodies HBsAg which is considered as a marker of chronic HBV infection. The magnitude of HIV/HBV coinfection among people living with HIV in Tanzania is not well known. Objective: The aim of this study was to determine the prevalence and associated factors of HBsAg among HIV Positive Clients at CTC at Mawenzi Regional Hospital. Methods: It was a cross sectional study that included 100 HIV Positive CTC clients at Mawenzi Regional Hospital. Ethical clearance was obtained at KCMUCo ethical committee, structured questionnaires with closed-ended question were used to collect the information needed done by interviewing the person, blood sample was collected from median cubital vein and HBsAg Rapid Test Strips were used. Data processing and analysis were done using SPSS version 20. Results: Of 100 HIV patients, only 8 (8%) tested positively for HBsAg. There was a significant association between residence and HBV infection. People from rural areas had high prevalence compared to urban areas (OR 8.71, 95% CI: 1.029 - 73.66). Other social demographic and clinical characteristics in this study had no significant association with HBsAg positivity. Conclusion: Significant numbers of 8% HIV patients are HBsAg positive. HIV patients from rural residency are more likely to acquire HBV than Urban residents that showed significant association.