A Wattbike is a stationary cycle ergometer that was originally designed for cyclists to provide them with the most realistic cycling experience. However, Wattbikes are now frequently used by athletes of various sports such as rugby union as they provide an alternative off-feet training method that places less stress on the lower limbs. This reduction in the amount of load placed on the lower limbs during off-feet training becomes crucial in specific situations such as returning to play after a lower body injury. Currently, research surrounding the Wattbike is limited to its effects on different aspects of performance during cycling. As Wattbikes are used by athletes from a variety of field sports where over-ground sprinting is essential, it would be beneficial to understand what effects training on a Wattbike has on performance during over-ground sprinting. As such, the purpose of the current study was to determine the effects of repeated Wattbike sprints on lower body horizontal power and power endurance during over-ground sprinting in male rugby union players. Fourteen male rugby union players were assigned to one of three groups; control, Wattbike or treadmill. During baseline testing, the participants performed a 30-m sprint and a 2 x 20-m shuttle repeated-sprint ability test. Participants in the Wattbike group also performed a 6-s sprint test on a Wattbike. After the baseline testing, the participants in the Wattbike group completed two repeated-sprint training sessions a week on a Wattbike. The repeated-sprint training protocol involved three sets of five to eight, 5-s sprints at 80 to 95% of peak velocity. The treadmill group completed the same protocol as the Wattbike group but on a motorised treadmill. The participants in the control group did not take part in any repeated-sprint training over the four-week intervention period. All participants repeated the same testing procedure used during baseline testing following the four-week intervention. The Wattbike intervention resulted in meaningful improvements in lower body horizontal power and power endurance. This is evident through the increases in absolute peak horizontal power (Pmax) (166.03 ± 147.00 W), and relative peak horizontal power (Rel Pmax) (1.71 ± 1.57 W·kgˉ¹), and decreases in RSA fastest (RSAf), slowest (RSAs) and mean (RSAm) sprint times (-0.17 ± 0.20, -0.25 ± 0.19, -0.21 ± 0.17, respectively). Furthermore, the increases seen in the Wattbike group for Pmax and Rel Pmax were greater than that of the treadmill group (ES = 0.77 ± 0.64 and 0.58 ± 0.50, respectively) and the changes in RSAf, RSAs and RSAm were substantially better than that of the control group (ES = ­1.35 ± 0.85, ­1.34 ± 0.87 and ­1.17 ± 1.00). The results of this study suggest that repeated Wattbike sprints are an effective method for increasing lower body horizontal power and power endurance during over-ground sprinting, though further research is still warranted.