The current study aims to (1) survey the association of common KCNJ11 polymorphisms (rs5210, rs5215, and rs5219) with T2D risk in a large sample size of the Iranian population; (2) examine pancreatic-cell function in individuals with different rs5219 genotypes; and (3) integrate our findings with all previous case–control studies and present the most comprehensive and up-to-date meta-analysis to precisely uncover the impact of common KCNJ11 polymorphisms on type 2 diabetes predisposition; (4) identify the probable sources of heterogeneity among studies used for the meta-analysis. The gene discussed is KCNJ11; the disease is type 2 diabetes mellitus.