BACKGROUND: Non-alcoholic fatty liver disease (NAFLD) is currently one of the most prevalent liver diseases. NAFLD is exemplified by the deposition of fat in the liver, in the absence of other etiologies. The spectrum of histological features in NAFLD ranges from macro-vesicular steatosis and nonalcoholic steatohepatitis, and it can eventually end in fibrosis, cirrhosis, or hepatocellular carcinoma. Vitamin D deficiency (VDD) is the most common micronutrient deficiency worldwide. Obese subjects are more prone to VDD, particularly those with liver disease. Non-classic functions of Vitamin D may be involved in the metabolic pathways beyond NAFLD development and progression. AIM: The aim of the study was to evaluate the relationship between NAFLD and Vitamin D levels in extreme obese subjects, and the correlation between Vitamin D levels and NAFLD severity. MATERIALS AND METHODS: The study included 80 Egyptian subjects of both sexes, divided into two groups: 50 patients with Stage III obesity (defined as body mass index [BMI] ≥40 kg/m2) and NAFLD, their age ranging from 20 to <60 years, and 30 age- and sex-matched healthy volunteers as control subjects. All patients were recruited from nutrition outpatient clinic at endocrinology unit, Cairo University Hospitals during the period from January 2019 to June 2019. Diagnosis of NAFLD was done by ultrasonography and laboratory evaluation included Vitamin D level. Nutritional evaluation included BMI, waist circumference, and weight. RESULTS: Vitamin D was significantly lower in the NAFLD group versus healthy controls: About 34%, deficient, 32% insufficient, and 34% sufficient in NAFLD group versus 23.3% insufficient, and 76.7% sufficient only in control, p ˂ 0.001. The severity of NAFLD, as graded by ultrasonography, was positively correlated with BMI and inversely correlated with Vit D levels, p = 0.001 and 0.024, respectively. Multivariate linear regression proved that both BMI and Vit D were independent predictors for NAFLD progression, BMI in a positive manner and Vit D in a negative manner. A cutoff Vit D level of 27.75 had 64% sensitivity and 90% specificity in NAFLD detection, area under the curve was 0.821. CONCLUSION: Vitamin D is significantly lower in the NAFLD group versus healthy controls in this cohort. VDD and BMI were associated with increased NAFLD severity. VDD was found to be an independent predictor of NAFLD progression. Vitamin D supplementation may be added to lifestyle modifications to prevent NAFLD occurrence in obese subjects.