The effects of whole grain highland barley (WGH) with rich phenolics on glucose metabolism, the insulin pathway, and microRNA (miRNA) expression in db/db mice were explored in the present study. Supplementation with WGH decreased the levels of blood glucose, glycosylated serum protein (GSP), insulin, and inflammatory cytokines in db/db mice. Furthermore, WGH administration triggered a remarkable amelioration of glucose intolerance and insulin resistance. The hepatic glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxylase (PEPCK) activities and the G6PC, PEPCK, and forkhead transcription factor 1 (FOXO1) mRNA levels in the WGH-treated group were also reduced. Moreover, WGH promoted the glycogen storage in the liver via up-regulating the activities of hexokinase (HK) and glycogen synthase (GS) and the phosphorylation of glycogen synthase kinase 3β (GSK3β) protein, while down-regulating the GSK3β mRNA level. The protein expression of phosphatidylinositol 3-kinase (PI3K), the phosphorylation of protein kinase B (Akt), and the mRNA levels of insulin receptor substrate-1 (IRS-1), PI3K and Akt were also up-regulated by WGH treatment. Moreover, WGH significantly augmented the expression of miRNA-26a and miRNA-451, but reduced those of miRNA-126a and miRNA-29a. These results demonstrated that WGH exhibits a hypoglycemic effect through regulating the IRS-1/PI3K/Akt pathway and related miRNAs, further modulating the expression of G6PC, PEPCK, and FOXO1 mRNAs and p-GSK3β protein, thus inhibiting hepatic gluconeogenesis, improving glycogen synthesis and alleviating insulin resistance. Therefore, this study suggested WGH as an effective candidate to ameliorate the hyperglycemia of type 2 diabetes mellitus.