Background: Gymnocarpos decandrus (Caryophyllaceae) is a well-known wild plant used as a food for grazing animals. Recently it showed potent antidiabetic potential beside its established anti-inflammatory, analgesic and diuretic activities. G. decandrus antidiabetic potential was reported through in-vitro models and resulted in promising α-amylase, α-glucosidase and antiviral Coxsackie B4 inhibitory activities; however no in-vivo studies were conducted.Purpose: This study aims to examine Gymnocarpos decandrus ethanol extract (GDEE) safety and to evaluate its in vivo antidiabetic potential.Method: Adult albino rats were injected intraperitoneally with alloxan to induce diabetes mellitus and the glucose level was measured after two and four weeks against metformin as a standard drug. Additionally, GDEE characterization and standardization were carried out.Results: GDEE LD50 was up to 5.8 mg/kg and exhibited significant antidiabetic activity 77.17% comparable to the standard drug metformin. Its total phenolics, and flavonoids amounted 127.2 ± 0.23 and 85.5 ± 0.21 mg/g respectively. Vitexin was used as a marker compound for GDEE (140.70 mg/100 gm).Conclusion: This study represents the sole in vivo scientific validation of G. decandrus recently documented in vitro antidiabetic potential.