The purpose of this study was to determine the effects of 12 weeks of circuit training on physical fitness in handball players. Subjects were randomly divided into a circuit strength training group (CT, n = 10) and a control group (CG, n = 9). Training sessions and matches were performed together, but during the 12-week intervention, the experimental group replaced part of the regular regimen with circuit strength training. Measures assessed in both groups before and after the intervention included: the agility T-half Test, the Yo-Yo intermittent recovery test, squat and counter-movement jumps, 15 m and 30 m sprints, and strength tests for the bench press, pull over, and the half squat. The upper limb bench press and pull-over tests along with the lower limb back half squat were performed using a 1-repetition maximum protocol. Based on the intraclass correlation coefficient and excluding the agility T-test (ICC = 0.72), we found excellent relative reliability for all variables (intraclass correlation coefficient range: 0.85-0.96, SEM range: 0.03-3.00). For absolute reliability or coefficients of variation, 71% (5/7) of the variables were excellent (CV < 5%). The circuit strength training group showed significant interaction effects and relevant effect sizes for the 12-week training period (8/9, 89%), and the mean effect size for the CT was markedly higher (d = 1.3, range: 0.41 - 2.76) than in the CG (d = - 1.0, range: -0.73 - 0.29). The largest improvements were in the Yo-Yo test (d = 2.76) and the squat jump (d = 2.05). These results show that a 12-week circuit strength training program is an effective method to increase handball-related performance characteristics.