Methionine synthase reductase (MTRR) gene is involved in tumorigenesis by regulating DNA methylation through activation of methionine synthase (MTR). MTRR is polymorphic at nucleotide 66 (A-to-G) and the resulting variant enzyme has a lower affinity for MTR. The reported associations of MTRR A66G polymorphism with cancer risk are contradictory. Therefore, we performed a meta-analysis to better assess the associations, including 18,661 cases and 27,678 controls from 35 studies. Crude ORs with 95% CIs were used to assess the strength of association between the MTRR A66G polymorphism and cancer risk. The pooled ORs were performed for homozygote model (GG vs. AA), heterozygote model (GG vs. GA), recessive genetic model (GG vs. GA + AA), and dominant genetic model (GG + GA vs. AA), respectively. Overall, results indicated that the G allele and GG variant genotypes were associated with a significantly increased cancer risk (G vs. A: OR, 1.039; 95% CI, 1.009-1.078; homozygote model: OR, 1.094; 95% CI, 1.006-1.191). In subgroup analysis by ethnicity, significant increased risks were found among Asians with G allele (G vs. A: OR, 1.063; 95% CI, 1.011-1.119; homozygote model: OR, 1.189; 95% CI, 1.055-1.341; recessive model: OR, 1.197; 95% CI, 1.068-1.341). For stratification analysis, the cancer types with fewer than three studies were categorized into "other cancers", and the results indicated that there was a significant elevated cancer risk in "other cancers" in all genetic models, not in colorectal cancer, lymphoid leukemia or breast cancer. In summary, our study suggests that the MTRR A66G polymorphism is a potential biomarker for cancer risk.