This study assessed the impact of 8 weeks biweekly in-season weightlifting training on the strength, throwing ability, and body composition of healthy male handball players. Twenty players (age: 21.2 ± 0.7 years, height: 1.83 ± 0.08 m, body mass: 83.3 ± 7.5 kg, body fat: 13.2 ± 1.4%, upper limb muscle volume: 3.16 ± 0.16 L) were randomly allocated between experimental (EG) and control (CG) groups. Measures of one-repetition maximal strength included bench press, pull-over, snatch, and clean and jerk. Throwing velocity was investigated by standing, running, and jump throws, and the power of the upper limbs was estimated from the total distance of a 3-kg medicine ball overhead throw. Muscle volumes were estimated anthropometrically. Training sessions comprised 3-4 sets of explosive weightlifting exercise at 75%-90% of 1RM (repetition maximum). Significant interaction effects (time x group) were found for all strength and throwing variables, ranging from η = 0.595 (pull-over) to η = 0.887 (medicine ball throw), with the largest between-group difference (more than 40%, Δd = 6.65) and effect size (d = 6.44) for the medicine ball throw, and the smallest (about 23%, Δd = 1.61) for the standing shot performance. Significant interaction effects were also detected for all anthropometric parameters (body mass: η = 0.433; body fat: η = 0.391; upper limb muscle volume: η = 0.920, with an almost 20% gain of muscle volume). It can be concluded that 8 weeks of biweekly in-season weightlifting training yielded substantial increases of muscle volume, maximal strength of the upper limbs, and ball throwing velocity in healthy handball players relative to their standard training program.