This study aimed to examine the effects of 12 weeks of strength training (ST) on sleep quality and daytime sleepiness in adolescents with sleep complaints. Thirty adolescents were randomly assigned to 2 groups: ST group (ST, n 5 18) and control group (CG, n 5 12). Anthropometric, body composition, one-repetition maximum test, and sleep parameters (Pittsburgh Sleep Quality Index [PSQI] and Epworth Sleepiness Scale [ESS]) were evaluated. Training consisted of 55 min•d 21 (3 times a week, for 12 weeks), 3 sets of 10-12 repetitions with a 1-minute rest interval between sets and exercises. Baseline and postintervention differences were analyzed using Generalized Estimating Equations and the effect size (ES) with Cohen's d coefficient. Significance was set at (p , 0.05). After 12 weeks of ST, a significant decrease in the PSQI score (7.3 6 0.7 vs. 5.1 6 0.6; ES 5 4.10) was observed in the ST group, but not in the CG (6.3 6 0.8 vs. 7.4 6 0.7; ES 5 1.53). A significant decrease in ESS score was found in the ST group (10.1 6 0.7 vs. 8.2 6 0.7; ES 5 3.08), without differences in the CG (10.7 6 0.8 vs. 11.0 6 0.7; ES 5 0.56). The ST group presented increased total sleep duration (h•min 21 ) (6.2 6 0.2 vs. 6.9 6 0.2; ES 5 3.60), but not the CG (7.0 6 0.2 vs. 6.8 6 0.1; ES 5 1.32). Individual analyses showed 67% of adolescents experienced a reduction in PSQI (8.3; confidence interval [CI] 95% 6.8-10.1) and ESS (8.3; CI 95% 6.7-9.9) scores after ST, whereas only 17% of control subjects presented reduced scores (PSQI [11.1; CI 95% 9.5-12.9] and ESS [11.0;). Strength training improved sleep quality and increased total sleep duration.