In this study we determined the nutraceutical potential of Juglans regia L. a diet supplement and food additive by assessing the in vivo anti-diabetic, anti-hyperlipidaemic and organ protective effects of dry powder of Juglans regia leaves in rats and the in vitro anti-diabetic potential of three Juglans regia leaf extracts. In the in vivo experiments, dry powder of Juglans regia leaf (25, 50 and 100 mg/kg) was administered orally, twice daily (9.00 a.m and 5 p.m) to streptozocininduced diabetic rats over a period of 28 days, during which body weight and blood glucose were monitored weekly. Phytochemical evaluation of three extracts was assessed using HPLC-PDA validated procedures, while enzyme inhibitory potentials were tested against α-amylase and α-glucosidase. Results revealed presence of significant quantities of rutin, catechin and 2,3-diMeO benzoic acid and inhibition of α-amylase and α-glucosidase activity. In vivo assays showed that twice-daily administration of increasing doses of J. regia leaf resulted in weight gain, glycemic control, reversal of dyslipidaemia, biochemical parameters of liver/kidney injury and protection against pancreas, liver and kidney injury. In conclusion J. regia L. leaves contain active phytochemicals that make it useful in the management of diabetes mellitus.