Background The relationship among vitamin D (VD) deficiency, metabolic syndrome, and type 2 diabetes mellitus (T2DM) is still doubtful, whether it is causal or jumbling. Aim To assess the link between VD deficiency and T2DM in old obese Egyptian patients and to evaluate the correlation between VD and some variables in T2DM. Patients and methods The study was conducted in Internal Medicine and Clinical Pathology Departments, Zagazig University Hospitals, from 02/2018 to 10/2018, involving 60 patients, aged more than or equal to 60 years, comprising 18 males and 42 females, who were divided into two groups: 30 patients with T2DM and 30 patients were proved to be without diabetes during the sample collection. Each group was subdivided according to their BMI into obese and nonobese. Results A highly significant difference was detected in VD level between nondiabetes and diabetes groups (23.85±12.1 vs. 16.2±7.9 ng/ml, respectively, P=0.001). VD deficiency was significantly higher in diabetes group (χ2=10.38, P=0.006). Insignificant difference was detected regarding age (67.3±2.4 vs. 67.7±2.5, P=0.5) and BMI (30.7±5.1 vs. 32.7±5.3 kg/m2, P=0.14) between nondiabetes and diabetes. There was a highly significant difference in VD level between obese nondiabetes and obese diabetes (27±17.5 vs. 13.3±8.2 ng/ml, respectively, P=0.001). A positive correlation was detected between VD level and fasting blood sugar (r=0.438, P=0.032), whereas a negative correlation between VD level and BMI in diabetes group only (r=−0.437, P=0.033) and in obese group only (r=−0.515, P=0.012). Conclusion Approximately 78.3% of patients had hypovitaminosis D (51.7% deficiency and 26.6% insufficiency). VD deficiency was significantly higher in diabetes group and in obese diabetics.