The current chapter emphasises the role of vitamin D in various autoimmune diseases and its immunomodulatory effects. Vitamin D, primarily known for its role in calcium homeostasis, has been found to interact with the vitamin D receptor (VDR) located on various immune cells, influencing both innate and adaptive immune responses. Through its action on dendritic cells, T cells, B cells, and macrophages, vitamin D promotes an anti-inflammatory environment by inhibiting proinflammatory cytokines and enhancing regulatory T cell function. Experimental evidence had also suggested that vitamin D deficiency correlates with increased severity of autoimmune disorders such as multiple sclerosis, type I diabetes mellitus, and rheumatoid arthritis. Clinical studies exploring vitamin D supplementation in autoimmune patients have shown promising but varied results, indicating its potential as an adjunct therapy. Further research is needed to elucidate the relationship between vitamin D deficiency and autoimmunity, so that the current therapeutic strategies for these complex diseases may further be improved for better outcomes.