The purpose of the current study was to examine the interactive effects of punishment and reward sensitivity in predicting Mentally Tough behaviour and performance in swimming. First, we validated a measure of MT behaviour in a mixed sample of competitive swimmers and then examined the interactive effects of punishment and reward sensitivities in predicting MT behaviour. A second purpose of the study was to examine whether punishment and reward sensitivities can account for race time performance. Results found significant interactions between reward and punishment sensitivity upon both MT behaviour and performance. That is, as punishment sensitivity increased MT and race times improved when reward sensitivity was low. However, both decreased when reward sensitivity was high. Results add to previous research showing that athletes who are sensitive to punishment and insensitive to reward display stronger MT behaviours and as a consequence, swim faster.