A total of 288 crossbred (Duroc${ imes}$Landrace${ imes}$Yorkshire) growing pigs were used in two experiments to investigate the effects of adding ${eta}$-mannanase to corn-soybean meal-based diets on pig performance and apparent total tract digestibility (ATTD). Both experiments lasted 28 d and were split into two phases namely 1 to 14 days (phase 1) and 15 to 28 days (phase 2). In Exp. 1,144 pigs weighing $23.60{pm}1.59$ kg BW were assigned to one of four corn-soybean meal-based diets containing 0, 200, 400 or 600 U/kg ${eta}$-mannanase. Increasing the level of ${eta}$-mannanase increased weight gain (quadratic effect; p<0.01) and feed efficiency (linear and quadratic effect; p<0.01) during the second phase and the overall experiment. However, performance was unaffected (p>0.05) by treatment during phase 1. Increasing the amount of ${eta}$-mannanase in the diet improved (linear and quadratic effect; p<0.05) the ATTD of CP, NDF, ADF, calcium, and phosphorus during both phases. Based on the results of Exp. 1, the optimal supplementation level was determined to be 400 U/kg and this was the level that was applied in Exp. 2. In Exp. 2, 144 pigs weighing $23.50{pm}1.86$ kg BW were fed diets containing 0 or 400 U/kg of ${eta}$-mannanase and 3,250 or 3,400 kcal/kg digestible energy (DE) in a $2{ imes}2$ factorial design. ${eta}$-Mannanase supplementation increased (p<0.01) weight gain and feed efficiency while the higher energy content increased (p<0.01) feed intake and feed efficiency during both phases and overall. Increased energy content and ${eta}$-mannanase supplementation both increased (p<0.05) the ATTD of DM, CP, NDF, ADF, phosphorus, and GE during both phases. There were no significant interactions between energy level and ${eta}$-mannanase for any performance or digestibility parameter. In conclusion, the ${eta}$-mannanase used in the present experiment improved the performance of growing pigs fed diets based on corn and soybean. The mechanism through which the improvements were obtained appears to be related to improvements in ATTD.