In the present study, a novel pentapeptide LPLLR (Leu-Pro-Leu-Leu-Arg) was purified and identified from walnut protein hydrolysates (3–10 kDa) by SEC, RP-HPLC, and LC-MS/MS. Its inhibitory actions against α-glucosidase and α-amylase were evaluated. Then its beneficial effects on glucose metabolism and the underlying molecular mechanism were determined in high glucose-induced insulin resistant (IR) hepatic HepG2 cells. Results demonstrated that LPLLR could inhibit both α-glucosidase and α-amylase, and the inhibition rate reached maximum of 50.12% and 39.08%, respectively, at concentration of 2000 μM. Moreover, LPLLR (100 and 200 μM) elevated the phosphorylation levels of IRS-1, PI3K, Akt, AMPK and GSK3β, as well as the expression levels of GS and GLUT4, while reduced the expression levels of G6Pase and PEPCK. These data revealed the mechanism of LPLLR in improving hepatic IR, which increased glycogen synthesis and glucose uptake, as well as decreased gluconeogenesis via activating the IRS-1/PI3K/Akt and AMPK signal pathways.