Background: Today obesity and overweightness are the major health problem all over the world. The relation between decreasing serum level of vitamin D and some chronic diseases such as diabetes and obesity was shown in the available studies. The aim of this study was to determine the prevalence of vitamin D deficiency among obese and overweight Iranian population and its relation with age, sex, occupation, body fat percentage and body weight according to body mass index. Methods: In a descriptive cross-sectional study, all patients who were referred to obesity clinic of Imam Khomeini Hospital, Tehran, for weight management throughout one year were included to the study. The prevalence of vitamin D deficiency with serum level lower than 30 ng/ml among 107 subjects was evaluated. The data were analyzed by SPSS, ver. 19 (SPSS, Inc., Chicago, IL, USA). Quantitative variables were presented as mean±SD and qualitative variables were presented as relative frequency. The groups were compared with independent samples t-test and Chi-square test. The level of significance for comparing the groups was considered at P<0.05. Results: In 107 participants, vitamin D deficiency in 81 subjects (75.7%) was reported. The prevalence of deficiency in participants with body mass index between 25 to 29.9, 30 to 34.9 and equal or greater than 35 kg/m2 was 77.3%, 83.7% and 69.4%, respectively. However, the differences between overweight and obese groups were not significant (P=0.193). The prevalence of moderate and severe vitamin D deficiencies among the participants was 50.5% and 25.2 %, respectively. Vitamin D deficiency was more common in participants below 50 years of age compared with those older than fifty (P=0.001). Conclusion: The prevalence of vitamin D deficiency was high in obese and overweight population especially among individuals younger than 50 years. It seems that, it is necessary to add prevention and management of this deficiency to obesity treatment protocols especially in the young adults.