Abstract Background and Aims Dysregulated T cell responses contribute to the pathogenesis of inflammatory bowel disease IBD. Because vitamin D vitD deficiency is a risk factor for adverse disease outcomes, we aimed to characterize the impact of vitD on intestinal and peripheral T cell profiles. Methods T cells were isolated from peripheral blood and intestinal biopsies of IBD patients, incubated with vitD and characterized by flow cytometry. To translate these in vitro findings to the clinic, serum vitD concentrations and clinical outcomes were correlated with T cell phenotype and function in a prospective patient cohort. Results Incubation of peripheral and intestinal T cells with 1,25(OH)2-vitD resulted in strongly reduced frequencies of pro-inflammatory CD4+ and CD8+ T cells producing interferon γ IFNγ, interleukin-17 IL-17, IL-22, IL-9 and tumour necrosis factor TNF. Univariable analysis of 200 IBD patients revealed associations of vitD deficiency with non-compliant vitD intake, season of the year and anaemia in Crohn’s disease CD as well as disease activity in ulcerative colitis UC. Ex vivo immunophenotyping revealed that CD4+ and CD8+ T cell subsets were not substantially altered in vitD-deficient vs vitD-sufficient patients while regulatory T cell frequencies were reduced in UC and non-smoking CD patients with vitD deficiency. However, normalization of serum vitD concentrations in previously deficient CD patients resulted in significantly reduced frequencies of CD4+ T cells producing IFNγ, IL-17 and IL-22. Conclusion vitD exerts profound anti-inflammatory effects on peripheral and intestinal CD4+ and CD8+ T cells of IBD patients in vitro and inhibits TH1 and TH17 cytokine production in CD patients in vivo.