The aim of this study was to evaluate the effects of dietary supplementation of bamboo leaf extract (BLE) on the growth performance, antioxidant traits, immune function, and lipid metabolism of weaning piglets. After weaning at 21 days, two hundred and forty healthy piglets (Large White × Landrance) were randomly assigned to 4 treatments with 6 pens and 10 piglets per pen. The control group (Ctr) received a maize-soyabean based diet, and the test groups received the control diet plus 0.5%, 1%, or 2% (w/w) BLE, respectively. The experiment was carried out for 5 weeks. At the end of it, average daily feed intake in the 1% BLE group was decreased (p < 0.05). Plasma concentrations of malondialdehyde were decreased with supplementation of 1% and 2% BLE. Immunoglobulin G concentrations and lysozyme activity in plasma were significantly increased in piglets supplemented with BLE. Diets with 1% and 2% BLE increased (p < 0.05) plasma concentrations of low density lipoprotein-cholesterol and reduced (P < 0.01) high density lipoprotein-cholesterol levels. Higher triglyceride concentrations were observed in the 0.5% (P < 0.01) and 2% (p < 0.05) BLE groups. In conclusion, these novel findings demonstrate that supplementation of BLE to the diet improved the antioxidant activity, immune function, and lipid metabolism of weaning piglets.