Background: Practitioners of traditional medicine use the decoction of Ononis natrix L. to treat hyperglycemia. The literature offers no evidence to support the use. Objective: To investigate the effect of the decoction of Ononis natrix L. on the blood glucose concentration in Wistar rats (Rattus norvegicus) with streptozotocin-induced diabetes mellitus. Methods: We obtained 35 Wistar rats from the animal colony of The University of Jordan School of Medicine. We induced diabetes by a single intraperitoneal injection of streptozotocin (60 mg/kg body weight) and 23 rats (66%) survived to allocation. We randomly assigned the rats to one of four groups: negative control (1% Tween 80 in distilled water), positive control (100 mg/kg metformin), high-dose treatment (7.5 mL of the decoction), and low-dose treatment (3.5 mL of the decoc-tion). We administered the doses twice daily by oral gavage for two weeks and measured the tail-blood glucose concentration twice daily, once before the first dose and another time after the second dose. We used linear mixed-effects regression to model the change in blood glucose concentration as a function of the experimentation groups, with adjustments for pseu-doreplication and temporal variation. Results: The estimated mean change was 1 mmol/L (−30 to 31 mmol/L) for the negative control group, −26 mmol/L (−56 to 5 mmol/L) for the positive control group, −75 mmol/L (−108 to −42) for the low-dose treatment group, and −82 mmol/L (−111 to −53 mmol/L) for the high-dose treatment group. Conclusion: In conclusion, we demonstrate, for the first time, the hypoglycemic effect of Ononis natrix L. in an animal model of diabetes.