Rheumatoid arthritis is an autoimmune disease that causes joint pain and reduces daily activities. The aim of this study was to evaluate the serum of vitamin D and its relationship with the severity of the disease in patients with rheumatoid arthritis referred to Allameh Hehlool hospital in Gonabad. This cross-sectional-analytical study was performed on 92 patients referred to the rheumatology clinic of Allameh Behlool Gonabad Hospital in 2021. After obtaining the permission of the ethics committee, the samples were selected based on the desired criteria. Serum vitamin D levels in patients were measured, and data were collected using a patient information checklist and DAS28-CRP activity questionnaire. Data were analyzed using SPSS software version 16 and using statistically appropriate tests at a significance level less than 5%. The mean age of the patients was 53.05 ± 12.33 years and most of them (58.7%) were women. Serum vitamin D level was sufficient in 65.2% of patients and the severity of the disease was in the remission in 48.9% of them. The results of chi-square test showed a significant relationship between serum vitamin D levels and disease severity in patients (P < .001). Serum vitamin D levels were inversely related to disease severity and in most patients with severe disease severity, serum vitamin D levels were inadequate. vitamin D supplementation is recommended in patients with rheumatoid arthritis.