Background and Purpose: Moyamoya disease (MMD) is an idiopathic cerebrovascular occlusive disorder prevalent in Korea and East Asia. RNF213 was recently identified as a susceptibility gene for MMD. The aim of this study was to investigate Korean RNF213 variants in MMD and their genotype-phenotype associations. Methods: Direct sequencing of RNF213 was performed in 165 Korean MMD patients from 155 unrelated families. The association of RNF213 genotype with MMD risk was evaluated dfusing historical controls for comparison. Correlations between RNF213 genotype and phenotype were statistically analyzed. Results: The c.14429G＞A (p.R4810K) variant was identified in 125 of 165 MMD patients (75.8%). Most patients (112) were heterozygotes and 13 patients had two copies of the c.14429G＞A variant. A novel heterozygous variant c.12086A＞G (p.Q4029R) was found in one additional patient. The minor allele frequency of the c.14429G＞A variant was significantly higher in the MMD patient group (138/330, 41.8%) than in the control group (8/588, 1.36%) (p 0.001). The c.14429G＞A (p.R4810K) variant significantly increased the risk of MMD in Korean patients, with an odds ratio of 52.11 (p 0.001) when compared to controls. The minor homozygote was highly associated with early onset MMD (age at onset 5 years), cerebral infarction at diagnosis, and cognitive impairment. Conclusions: The c.14429G＞A (p.R4810K) variant of RNF213 is strongly associated with early onset MMD in Korean patients and this genotypic variant may be a useful biomarker for young age onset MMD or MMD with cerebral infarction.