Background: Given the variations in clinical presentation and physiopathological mechanisms in irritable bowel syndrome (IBS) subtypes, it is an acknowledged fact that the response to treatments can be disparate. Objective: To assess the effect of vitamin D on inflammatory cytokines (IL-17, IL-10, TNF-α), and biomarkers of oxidative stress (total antioxidant capacity (TAC), and malondialdehyde (MDA)) among IBS patients. Methods: A double-blind, randomized, placebo-controlled 6-month intervention study was carried out on 90 IBS patients (85 were analyzed), as defined by the Rome III criteria. Study participants were randomly assigned to receive either 50,000 IU vitamin D3 or a placebo fortnightly. Results: Vitamin D supplementation significantly reduced the IL-17 and MDA serum levels (P<0.05) and observably increased the TAC and IL-10 serum levels (P<0.05), compared with the placebo group. Comparing different bowel habit subtypes, we observed that it was only in diarrhea predominant IBS (IBS-D) that vitamin D supplementation was able to significantly reduce the serum levels of TNF-α and IL-17 (P<0.05). However, in all subtypes, IL-10 and TAC increased, while MDA decreased (P<0.05) in vitamin D group, compared to the placebo group. Conclusion: Vitamin D3 supplementation reduces the serum IL-17 and MDA levels, and augments the serum IL-10 and TAC levels in IBS patients, particularly in IBS-D subtype. Thus, the present study demonstrates the beneficial effects of vitamin D on patients with IBS-D.