Background: Vitamin D insufficiency plays an important role in the development of fibrosis in chronic liver disease. Methods: This was a cross-sectional study from Thailand. Liver fibrosis was assessed by transient elastography. Serum 25 hydroxyvitamin D (25[OH]D)<30 ng/ml was defined as hypovitaminosis D. 25(OH)D was assessed prior to and following tenofovir disoproxil fumarate (TDF). Factors related to 25(OH)D levels were determined by logistic regression analysis. Results: A t otal of 158 HIV-HBV-coinfected patients (32% female, median age 43 years) were included. Overall, liver disease was mild with 13.4% having a fibrosis score (FS) of 7.1-14 kPa and 2% with a FS>14 kPa. Median (IQR) duration on TDF was 5 years (4-7). The median estimated glomerular filtration rate was 96.9 ml/min/1.73 m2. The median (IQR) serum 25(OH)D levels prior to and following TDF were 24.8 ng/ml (21.3-30.6) and 22.8 ng/ml (18.0-27.7), respectively; P≤0.001). The proportion of patients with hypovitaminosis D significantly increased from 72.2% (95% CI 64.7, 78.6) prior to TDF to 84.2% (95% CI 77.7, 89.0) after taking TDF (P=0.01). Factors associated with hypovitaminosis D by multivariate analysis were female sex (adjusted OR 3.8, 95% CI 1.1, 13.7; P=0.038) and duration of antiretroviral therapy (ART)>5 years (OR 3.3, 95% CI 1.2, 8.8; P=0.017). Vitamin D levels were not associated with significant liver fibrosis. Conclusions: Although our HIV-HBV-coinfected patients live in the tropics, there was a high prevalence of hypovitaminosis D, especially in female patients and those receiving prolonged ART. Since HIV-HBV-coinfection requires long-term use of the HBV-active drug, TDF, which can also contribute to bone loss, routine vitamin D assessment and supplementation as necessary should be considered.