Background: Diabetes mellitus is a chronic metabolic disorder characterized by hyperglycaemia and other symptoms like polyuria (frequent urination), polydipsia (increased thirst) and polyphagia (increased hunger) which ultimately causes various other complications like retinopathy, neuropathy, nephropathy and microangiopathy. Objectives: The antidiabetic and antihyperlipidemic potential of oil from Piper longum (PLO) and piperine was investigated with their possible mechanism using α-glucosidase, aldose reductase (AR), and pancreatic lipase inhibitory activity. Methods: The biochemical parameters, viz. glucose level, insulin level, liver glycogen content, glycosylated hemoglobin, total plasma cholesterol, triglyceride, and antioxidant parameters, were estimated for all treated groups in acute and chronic antihyperglycemic animal models. Results: PLO (100 and 200 mg/kg), piperine (25 and 50 mg/kg), and glibenclamide (0.6 mg/kg) in respective groups of diabetic animals administered for 28 days reduced the blood glucose level in streptozotocin-induced diabetic rats. There was significant increase in body weight, liver glycogen content, plasma insulin, and high-density lipoprotein and decrease in glycosylated hemoglobin, triglyceride, and total plasma cholesterol in PLO-administered groups as compared to control group. The IC50 value of PLO for α-glucosidase, AR, and pancreatic lipase was found to be 150 ± 2.5, 120 ± 1.2, and 175 ± 1.2 μg/ml, respectively, which was found comparable with the standard drugs acarbose (90 ± 2.3 μg/ml), quercetin (80 ± 2.3 μg/ml), and orlistat (25 ± 0.5 μg/ml), respectively. Conclusion: The investigation done reveals that PLO has significant antidiabetic and antihyperlipidemic activity.