OBJECTIVES: This study aimed to evaluate the relationship between the 25-hydroxyvitamin D [25(OH)D] level and rheumatoid arthritis (RA) and the correlation between serum vitamin D level and RA activity. METHODS: We searched the PUBMED, EMBASE, and Cochrane databases and performed a meta-analysis examining the vitamin D level and prevalence of vitamin D deficiency in patients with RA compared to healthy controls and the correlation coefficients between the vitamin D level and disease activity score 28 (DAS28) in RA patients. RESULTS: Fifteen studies that included a total of 1,143 RA patients and 963 controls were available for this meta-analysis. The meta-analysis showed that the serum vitamin D level in the RA group was significantly lower than that in the control group (SMD=-0.608, 95% CI=-1.105-[-0.017], p=0.017). In addition, the prevalence of vitamin D deficiency was significantly higher in the RA group than in the control group (55.2% vs. 33.2%; OR = 2.460, 95% CI = 1.135-5.332, p=0.023). Thirteen studies evaluated the correlation between the vitamin D level and its activity in 924 RA patients. Meta-analysis showed a significant inverse correlation between the vitamin D level and DAS28 (Correlation coefficient =-0.278, 95% CI =-0.393-[-0.153], p=1.8 x 10-5). CONCLUSIONS: Our meta-analysis demonstrates that serum vitamin D level is significantly low in patients with RA, vitamin D deficiency is prevalent in RA patients compared to controls, and the vitamin D level correlates inversely with RA activity. Our meta-analysis suggests that the vitamin D level is associated with susceptibility to RA and RA activity.