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Type 1 diabetes mellitus (T1DM) is an autoimmune disease characterized by the destruction of pancreatic beta cells. Vitamin D is known to have immunomodulatory effects that may influence T1DM; however, its impact on immune gene expression in T1DM patients remains unclear. This study aimed to explore the effects of vitamin D on immunomodulatory gene expression in T1DM patients through bioinformatics analysis and qRT‒PCR validation. Two microarray datasets (GSE55098 and GSE50012) were analyzed to identify differentially expressed genes (DEGs) in T1DM and vitamin D-treated samples. Enrichment analysis and protein‒protein interaction (PPI) network analysis were used to identify key immune genes. qRT‒PCR was then performed on peripheral blood mononuclear cells (PBMCs) from T1DM patients and healthy controls. The analysis revealed 100 DEGs shared between the two datasets, with 58 showing opposite regulation. Six key immunomodulatory genes (CD226, CD40, RSAD2, OAS3, PTGS2, and LAMP3) were identified. qRT‒PCR confirmed the significant dysregulation of these genes in T1DM patients. Vitamin D treatment led to the downregulation of most genes, except for LAMP3, whose expression remained unchanged. Vitamin D modulates the expression of key immune genes in T1DM, suggesting its potential as an adjunctive therapy. Future studies should validate these findings with larger cohorts and explore the long-term effects of vitamin D on immune regulation.