Background: Globally, medicinal plants are used to treat diseases like diabetes. The present study evaluates the possible antioxidant, acute oral toxicity, the in-vitro and in-vivo antidiabetic potential of the hydro-ethanolic leaf extract of Koenigia polystachya (HELeKP) against beta-cell damage in experimentally induced diabetes mellitus. The DPPH (2,2-diphenyl-1-picrylhydrazine), ABTS [2,2′-azino bis-(3-ethylbenzothiazoline-6-sulfonic acid)], ­H2O2 (Hydrogenperoxide), superoxide radical scavenging activity and NO (Nitric oxide) assay estimated the in-vitro antioxidant assay of HELeKP. The acute oral toxicity study was evaluated per the OECD (Organization for Economic Cooperation and Development) test guidelines 425. Diabetes was stimulated in rats with a single dose of Streptozotocin (STZ), and after confirmation of diabetes, HELeKP was given orally for 21 days. Blood/serum samples were gathered and examined for biochemical changes, while tissue samples were evaluated for histopathological alterations. Results: The ­IC50 value of the HELeKP for all the anti-oxidant assays confirms the free radical scavenging activity. The data on acute oral toxicity revealed that the HELeKP used in the study was comparatively very safe. The outcomes of the in-vivo study suggested that the extract significantly reduced (p < 0.001) the fasting glucose level in STZinduced diabetic rats. Furthermore, the lipid profile level was significantly normalized (p < 0.01, p < 0.001) in diabetic rats. The histopathological observation of the pancreas in HELeKP-treated rats showed significant beta-cell restoration. Conclusions Based on the outcomes of this study, the HELeKP-treated rats have significant free radical scavenging and anti-diabetic potential. Therefore, it can be recommended as a beneficial functional vegetable for consumption.