Forty-eight (37.7 ± 3.4 kg, initial shrunk live weight) lambs were used in a 61 d experiment to evaluate the energy value of grease trap waste (GT) at four levels of supplementation (0%, 2%, 4%, and 6%). Supplemental GT replaced cracked corn in the basal diet. The GT contained 6.4% moisture, 3.1% impurities, and 79.8% total fatty acids (FA). Increasing GT level in diets did not affect dry matter intake and daily weight gain but linearly increased gain efficiency and estimated dietary net energy (NE). However, the ratio of observed-to-expected diet NE decreased with increased levels of GT. The estimated NE values for GT based on FA intake were in close agreement (98% and 102% of predicted, respectively) with those NE values determined by replacement technique for 2% and 4% supplementation level. However, the observed NE value for GT supplemented at the 6% level was 9% lower than predicted. Kidney–pelvic–heart fat increased as level of GT supplementation increased; otherwise, carcass characteristics and shoulder composition were not affected. We conclude that GT is a suitable alternative to conventional feed fats in diets for finishing lambs. The estimated NE of GT is 93% the energy value assigned by current standards for tallow and yellow grease.