Harries, SK, Lubans, DR, Buxton, A, MacDougall, THJ, and Callister, R. Effects of 12-week resistance training on sprint and jump performances in competitive adolescent rugby union players. J Strength Cond Res 32(10): 2762–2769, 2018—Sprint performance is an important characteristic for success in many sports, including rugby union. Resistance training is used to increase muscular fitness (i.e., strength, endurance, and power) and may also be effective for improving sprint and jump performances. The aims of this study were to examine the effects of resistance training using 2 different periodized programs (linear and daily undulating) on sprint and jump performances and explore relationships between performance measures. Sixteen male (16.9 ± 1.0 years) adolescent rugby union players participated in 12 weeks of resistance training. A further 10 male (15.5 ± 1.0 years) participants were recruited as a control group. Assessments of strength (box squat), 10- and 20-m sprint (electronically timed), and jump height (maximal unloaded (body mass only) and loaded (body mass + 10 kg) countermovement jumps) were conducted before and after 12 weeks training. Large to very large increases in 1 repetition maximum box squat (linear: 33.9%; p < 0.001; effect size (ES) = 1.64; daily undulating: 44.5%; p < 0.001; ES = 2.33) were observed after training. Small decreases were seen in 10-m (linear: −1.6%; p = 0.171; ES = −0.84; daily undulating: −2.5%; p = 0.038; ES = −0.36) and 20-m (linear: −0.5%; p = 0.506; ES = −0.20; daily undulating: −1.7%; p = 0.047; ES = −0.27) sprint times. Small-to-moderate associations between changes in lower-body strength and improvements in 10- and 20-m sprint times were found. Resistance training increases lower-body strength in adolescent rugby union players and increases in lower-body strength may transfer to improved sprinting performance with improvements after daily undulating periodized resistance training slightly superior.