Mouthguards are essential for preventing orofacial trauma in contact sports, such as rugby. However, comfort issues often hinder athlete compliance. Demonstrating an ergogenic effect could encourage greater use. This study aimed to examine the impacts of wearing a custom-made mouthguard on isokinetic shoulder strength, focusing on its velocity-dependent effects and its influence on inter-limb asymmetry. Twenty-nine male university rugby players performed maximal concentric isokinetic shoulder movements in a diagonal pattern (adduction and abduction) using a dynamometer. A randomized, crossover design compared performance with a custom-made mouthguard and without a mouthguard for both dominant and non-dominant limbs at angular velocities of 60°/s, 120°/s, and 180°/s. Key metrics included peak torque and rate of force development. Mouthguard use showed a significant main effect, with significantly higher peak torque and rate of force development compared to without a mouthguard (p