This study aimed to compare the effects of 5 weeks of unilateral and bilateral leg press training on lower body strength, linear sprinting and vertical jumping performance in adolescent rugby players. Twenty-six male adolescent rugby players (age = 15.3 ± 0.4 years) were assigned via stratified block randomization to unilateral (n = 9), bilateral (n = 9) and control (n = 8) groups. Training consisted of either the unilateral or the bilateral leg press twice weekly over five weeks, with the control group maintaining habitual training. Lower body unilateral and bilateral strength, vertical jump and linear sprint performance were assessed before and after training. After 5 weeks of training, both training groups significantly increased the 5-repetition maximum bilateral leg press (unilateral group = 8.9%, d = 0.53; bilateral group = 10.9%, d = 0.55, p < 0.01) and the 5-repetition maximum unilateral leg press (unilateral group = 20.2%, d = 0.81; bilateral group = 12.4%, d = 0.45, p < 0.01). There was no significant difference between the size of improvement in unilateral and bilateral groups in the 5-repetition maximum bilateral leg press, but the 5-repetition maximum unilateral leg press increased significantly more in the unilateral group (p < 0.05). No significant training effects were found for vertical jump or linear sprint performance. The results indicated that unilateral leg press training was as effective as bilateral leg press training in improving bilateral strength and more effective in improving unilateral strength in adolescent rugby players. However, strength improvement did not transfer to athletic performance improvements in either group