Athletes wear mouthguards for their protective value against trauma. Mandibular position has been linked to postural changes and improved athletic performance. This randomized double-blind comparative study aimed to evaluate the postural changes induced by two different mouthguards in athletes. Seventy-four volunteer varsity athletes, male and female, from soccer and hockey teams of the Université de Moncton were fitted with two mouthguards; Indented Midlined Stabilizing (IMS) and Flat (F). Using MatScan^® linked to BioEMG^®, sway parameters and surface electromyography of postural muscles were assessed for eyes closed static erect posture for three mandibular positions: habitual occlusion and the two mouthguards. The deltoid resistance was also assessed for the three conditions. There was significant stabilization of sway (p<0.0001) (area, distance, left-right excursion and antero-posterior excursion) by the IMS mouthguard compared to the F mouthguard and habitual occlusion. Those last two did not differ with respect to sway. There was no difference in the surface electromyographic recording under all three conditions. The deltoid resistance was improved only by the IMS mouthguard. Properly designed mouthguards provided postural stabilization and increased deltoid resistance for athletes. These observed positive changes in posture could potentiate athletic performances, play a role in injury prevention and have an impact in rehabilitation.