Background: Diabetes is a non-communicable disease causing impairment of carbohydrate and lipid metabolisms. Previous ethnobotanical surveys showed that leaves of Glyphaea brevis (G. brevis) are used conventionally in the treatment of diabetes with limited scientific evidence. Objective: We aimed to determine the effects of aqueous (AE) and hydroethanolic extracts (HEE) of G. brevis leaves in attenuating diabetes-linked hyperglycemia and hyperlipidemia in an animal model. Materials and Methods: Thirty streptozotocin-induced diabetic male rats were divided into six groups receiving the following daily treatments for 4 weeks orally: control (distilled water), reference (tolbutamide 80 mg/kg) and 4 tests (AE 250 mg/kg, AE 500 mg/kg, HEE 250 mg/kg, and HEE 500 mg/kg). The effects of each treatment on postprandial hyperglycemia were assessed using oral glucose tolerance and oral starch tolerance tests. Blood was collected to assess the effects of treatments on fasting plasma glucose, glycated hemoglobin (HbA1c), and lipid profile. Liver glycogen and gluconeogenic enzyme activity were also measured. Results: AE- and HEE-treated rats had 36%–64% lower fasting blood glucose levels, 34%–73% lower postprandial glycemia, and 15%–75% lower HbA1c than rats from control group (P < 0.01). AE and HEE treatments also brought about a significant increase in liver glycogen levels and lower gluconeogenic enzyme activity (P < 0.01). Extract-treated groups also had lower plasma total cholesterol and low-density lipoprotein-cholesterol concentrations (P < 0.01). Conclusion: These results suggest that treatment with leaf extracts of G. brevis is effective in attenuating hyperglycemia and hyperlipidemia in streptozotocin-induced diabetic rats. Future studies will determine the active compounds accountable for these beneficial effects.