Abstract Background Vitamin D has immunomodulatory effects in multiple sclerosis (MS). Vitamin D acts through various mechanisms such as secretion of cytokines. Interleukin-17 (IL-17) is a critical interleukin in inflammatory response in MS. Objective This study assessed the effect of oral high dose vitamin D intake on IL-17 levels in MS patients in a double blind randomized clinical trial. Methods 94 patients with a diagnosis of relapsing remitting multiple sclerosis (RRMS) were randomized to two groups. One group received 50,000 IU vitamin D3 every five days for 12 weeks. The other group was given placebo. Both groups received interferon-β (IFN-β) treatment. Serum levels of IL-17 were measured at the beginning of the study and after 12 weeks. Results IL-17 serum levels were 56.75 ± 28.72 pg/ml and 30.31 ± 75.85 pg/ml in the intervention and placebo group at the beginning of the study, respectively (Median ± IQR, p = 0.338). After 12 weeks, IL-17 levels were 58.93 ± 67.93 pg/ml and 46.13 ± 94.70 pg/ml in the intervention and placebo group, respectively (Median ± IQR, p = 0.960). The multiple linear regression analysis indicated that the consumption of vitamin D3 was positively and significantly associated with the logarithm of IL-17 measures (β = 1.719; p = 0.002 and R2 = 0.91), adjusted by EDSS scores. Conclusion IL-17 levels showed significant change in RRMS patients after receiving high dose vitamin D3 for 12 weeks.