Aim: The aim of this study was to assess the influence of custom-fit mouthguards on the aerobic performance of soccer and futsal players under 17 (U-17). Material and methods: Forty players from 3 Brazilian clubs participated in the study. The athletes' aerobic performance was assessed through the 20-meter shuttle-run test. All athletes performed two tests with and without mouthguard. Two outcome variables were analyzed: (i) the total distance covered in the test (meters) and (ii) the maximum oxygen uptake (VO2 max). A questionnaire assessing the level of acceptance of mouthguards considering different parameters was administered to the athletes before the delivery of the mouthguards and after 2 weeks of use. The questionnaire used a visual analogue scale (VAS). Paired t-test was used to compare the results obtained from the shuttle-run tests and the questionnaires. Results: Mouthguards did not influence the aerobic performance of the players, considering both the total distance covered in the tests and the VO2 max. Stability was the parameter with the highest acceptance. Levels of acceptance regarding breathing (P = 0.022) and communication (P = 0.002) increased after mouthguards usage. Communication had the lowest level of acceptance considering all parameters assessed. Only 10% of the players reported receiving recommendations to use mouthguards while playing football or futsal. None of the participants reported having used mouthguards before. Conclusions: The use of custom-fit mouthguards does not affect the aerobic performance of soccer and futsal U-17 players. Future studies should focus on the development of appliances with maximum protection and minimum influence on communication.