Introduction: Patients with ulcerative colitis (UC) can suffer from low serum vitamin D that can result in complications such as low bone mineral density. It can also reflect underlying disease severity. Methods: One hundred and ninety-seven patients previously diagnosed with UC from 2 European centers were prospectively recruited through the out-patient clinics. Clinical features (Montreal Classification, age, gender, previous and current medications, surgery), disease activity (Simple Clinical Colitis Activity Index [SCCAI]), blood investigations including serum inflammatory markers, and serum vitamin D were analyzed. The vitamin D levels were compared to a group of age- and gender-matched healthy controls. Results: Mean vitamin D levels were lower in patients with UC (54.6 nmol/L) than in controls (80.7 nmol/L; p = 0.0001). Mean vitamin D levels was lowest in patients with extensive UC (E3; p = 0.0001). Serum vitamin D was not significantly different across treatment groups (p = 0.876). There was no statistical difference in vitamin D levels across patients receiving calcium and vitamin D supplements (p = 0.35) and there was no statistical correlation with SCCAI (p = 0.22). Conclusions: This study confirms the existence of low serum vitamin D in patients with UC when compared to healthy controls. It also provides evidence of an existing relationship between disease extent and serum vitamin D.