Type-2 diabetes (T2D) is the most common type of chronic disease in adults and accounts for around 90% of all cases of diabetes. Therefore, developing dietary supplements from natural sources, such as teas, is of great interest. Seven diet groups together with a parallel control group were used for three periods of 16 weeks in total [stabilization period (W-2-W0), model period (W0-W8), and treatment period (W8-W14)]. The primary aim of this study was to investigate the in vivo antidiabetic activities of green and black tea polysaccharides (GTPS and BTPS, respectively) using streptozotocin induced diabetic mice fed with either a high-fat diet (HFD) or normal diet (ND). Streptozotocin and HFD induced T2D in vivo model was developed during the model period (W0-W8) in C57BL/6J male mice. Both GTPS and BTPS groups were administrated for 6 weeks (daily 400 mg/kg body weight) by oral gavage throughout the treatment period (W8-W14). The results showed that BTPS group significantly (P < 0.05) decreased the fasting blood glucose level in diabetic mice even fed with a HFD and improved the insulin resistance. Similar effect was not obtained when GTPS group fed with a HFD. In addition, BTPS group fed with a HFD effectively suppressed the body weight gain despite high energy intake and was more successful than its GTPS counterpart group in healing pathologies of liver and affected plasma blood lipid levels due to streptozotocin and HFD-induced diabetes. The present work suggests that BTPS can be used as an antidiabetic dietary supplement without posing any potential health risk.