Evidence has shown that liver disease caused by hepatitis viruses can be more aggressive and severe in HIV infected subjects. Therefore, the present cross-sectional study aimed to evaluate the seroprevalence of HDV infection among HIV/HBV co-infected clients in Shiraz, southwest Iran. In this study, 178 patients co-infected with HBV and HIV individuals were enrolled. The diagnosis of HIV infection was documented based on serological assays. The demographic and complementary data were collected by a questionnaire. HBsAg and HDV Ab were detected by commercial quantitative enzyme linked immunosorbent assay kits according to the manufacturer’s instructions. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were also measured. The mean age of the participants was 37.4±7.4 years (range 22-63). 175 (98.4 %) patients were male and 3 (1.6 %) were female. Among 178 patients co-infected with HIV/HBV, 35 cases (19.7%, 95% CI: 14%-25%) were anti-HDV positive and 143 (80.3%) were negative for anti-HDV. HDV exposure in HIV/HBV co-infected patients was associated with blood transfusion (P=0.002, OR: 14.3) and prison history (P=0.01, OR: 2.31) but not with age, marital status, unsafe sex contact, and injection drug abuse. Our data showed a relatively high prevalence of HDV infection in HIV infected population in Shiraz, Iran. The high frequency of HDV Ab in patients with blood transfusion and prison history reveals that HDV transmission occurs more frequently in the parental route than sexual contacts; therefore, blood screening for HDV diagnosis in the high-risk group is recommended.