Abstract – Objectives: The purpose of this study was to measure the comfort, wearability, physiological effects and its influence on athletes’ physical performance, of custom-fitted compared with self-adapted mouthguards (MGs). Methods: Eleven rugby players were put under specific efforts similar to those of the competition. Each player made three consecutive tests randomly wearing a commercially available ‘boil-and-bite’ self-adapted mouthguard (MG2), a custom-fitted mouthguard (MG3), and no mouthguard (reference). Forced expiratory air volume at 1 s (FEV1), expiratory flow rates peak (PEF), forced vital capacity (FVC), rebound (RB) jump 15 s, and counter-movement jump (CMJ) were measured on each player before and after the training exercise tests. Subjective evaluations by means of a visual analog scale (VAS) questionnaire took place. Comforts, adaptability, stability, tiredness, thirst, oral dryness, nausea, ability to talk, breathe, and drink were evaluated. Results: The wearing of the self-adapted MG showed significant improvement in PEF (P < 0.05). There were no statistically significance differences regarding the others spirometer parameters. In CMJ, there were no differences between both the MGs. On RB power was similar with both MGs and control. However, RB height reduced significantly wearing MGs. MG3 showed superior properties in comfort, adaptability, stability, and ability to talk and to breathe. Conclusions: MG3 showed the smallest range of changes in players’ performance, suggesting improved fit, comfort, and acceptation compared with MG2. Furthermore, its greatest advantage is the individualized design according to the proper anatomy of the oral cavity. Greater efforts must be made to improve the comfort of MGs if their use is to be increased.