Background: Vitamin D supplementation is recommended for patients with multiple sclerosis (MS). However, a recent meta-analysis based on low-quality trials suggested no evidence of supplementation benefit. A systematic review and meta-analysis of high-quality observational cohort studies should provide us further evidences. Methods: MEDLINE, EMBASE, and WEB-of-SCIENCE databases were systematically searched to identify eligible studies published before October 2018. Prospective cohort studies assessing the associations of serum 25(OH)D levels with MS relapses, radiological inflammatory lesions, or changes in expanded disability status scale in adults (≥18 years) with MS were included. Pooled RRs were calculated using fixed-effect or random-effects model depending on heterogeneity. Results: Thirteen studies and 3498 patients were included. Each 25 nmol/L increase in serum 25(OH)D levels was associated with a reduction in (1) clinical relapse rate [RR = 0.90; 95% confidence interval (CI) = 0.83-0.99], (2) gadolinium-enhancing lesions (RR = 0.69; 95% CI = 0.60-0.79), (3) new/enlarging T2 lesions (RR = 0.86; 95% CI = 0.77-0.95), and (4) new active lesions (RR = 0.81; 95% CI = 0.74-0.89) in the magnetic resonance imaging(MRI). Conclusions: Serum 25(OH)D levels are associated with a modest decrease in relapse rate and radiological inflammatory activities in patients with MS. The association with disability worsening remains inconclusive.