Background: Vitamin D plays a crucial role in bone formation, immune system modulation and cell differentiation. It could act as protective factor against autoimmune disease, in particular Rheumatoid Arthritis (RA). Objective: The aim of our study was to study the link between vitamin D deficiency and RA severity in Western Algeria. Methods: This cross-sectional study included 1022 RA patients with either vitamin D deficiency or insufficiency diagnosed between 2014 and 2024. Results: Among the RA patients, 746 were females and 276 males, divided according to vitamin D levels: 708 deficient, 165 insufficient and 149 with normal levels. The mean age of patients with vitamin D deficiency (54.215±12.311 years) was significantly higher (P<0.0001), with a female predominance (77.7%). As patients with vitamin D deficiency had higher disease activity scores, measured by Disease Activity Score 28 (DAS28), Simplified Disease Activity Index (SDAI) and Clinical Disease Activity Index (CDAI) (P<0.0001). Biological parameters such as Erythrocyte Sedimentation Rate (ESR), C-reactive protein (CRP), Anti-Cyclic Citrullinated Peptide ( Anti-CCP) and Rheumatoid Factor (RF) were also elevated (P<0.0001). Conclusion: The vitamin D deficiency, particularly in the western Algeria, is common and associated with a deterioration in biological and clinical symptoms, contributing to RA disease worsening.