Theasinensin A (TSA), a dimer of epigallocatechin gallate, has been preliminarily demonstrated to have hypoglycemia and anti-inflammatory effects. However, little information is available on its potential mechanisms of anti-diabetes. Therefore, the present study aimed to investigate the influence of TSA on glucose and lipid metabolism and gut microbiota in high-fat-diet/streptozotocin-induced diabetic mice. As result, TSA improved polydipsia, polyphagia and impaired glucose tolerance of diabetic mice, declined the fasting blood glucose and hepatic triglyceride level, and enhanced the expression at mRNA level of insulin receptor substrate, phosphoinositide 3-kinase, protein kinase B and glucagon-like peptide 1 receptor (GLP-1R) in the diabetic liver. Moreover, TSA could restore the disorder of gut microbiota of diabetic mice. High-dose (100 mg/kg) TSA showed better beneficial effects from the blood biochemical parameters, hepatic function and gut microbiota. In general, high-dose TSA significantly modulated gut microbiota by increasing the relative abundance of Akkermansia and decreasing the relative abundances of Acetatifactor, Anaerotruncus, Pseudoflavonifactor, Oscillibacter and Clostridium clusters. The results indicated that TSA could exert an anti-diabetes effect in diabetic mice through restoring glucose homeostasis, declining hepatic steatosis, activating insulin and GLP-1 signaling pathways, and ameliorating gut microbiota dysbiosis.