Purpose: Resistance training is one of the key components influencing power output. Previous studies directed at power development through the use of postactivation potentiation have analyzed resistance exercises at volitional or fast tempo of movement in the entire cycle, without control of the duration of the concentric and eccentric phases of movement. To date, no scientific studies have explored the effects of varied movement tempo on the level of power output, velocity, and postactivation potentiation efficiency. Methods: During the experimental sessions, study participants performed 3 sets (Sets1–3) of the bench-press exercise using 70% 1-repetition maximum and 2 different tempos of movement: 2/0/X/0 eccentric medium tempo (ECCMED) and 6/0/X/0 eccentric slow tempo (ECCSLO). Results: Post hoc analysis demonstrated significant differences in values of peak (P PEAK) and mean (P MEAN) power between Sets1–3 measured for the ECCMED (2/0/X/0) tempo. The values of P MEAN in Set3 (492.15 [87.61] W) were significantly higher than in Set2 (480.05 [82.10] W) and Set1 (467.65 [79.18] W). Similarly, the results of P PEAK in Set3 (713.10 [132.72] W) were significantly higher than those obtained in Set2 (702.25 [129.5] W) and Set1 (671.55 [115.79] W). For the ECCSLO tempo (6/0/X/0) in Set2 (587.9 [138.48] W), the results of P PEAK were significantly higher than in Set1 (565.7 [117.37] W) and Set3 (563.1 [124.93] W). Conclusions: The results of this study indicate that the postactivation potentiation effect is observed for both slow and medium tempos of movement.