Wearing an intraoral jaw-protruding splint could enhance respiratory function in clinical settings and eventually exercise performance. Purpose: The authors studied the acute effect of wearing a lower-jaw-forwarding splint at different protruding percentages (30% and 50%) across a wide range of running exercise intensities. Methods: A case study was undertaken with a highly trained and experienced 27-year-old female triathlete. She performed the same incremental intermittent treadmill running protocol on 3 occasions wearing 3 different intraoral devices (30% and 50% maximum range and a control device) to assess running physiological and kinematic variables. Results: Both the 30% and 50% protruding splints decreased oxygen uptake and carbon dioxide production (by 4%–12% and 1%–10%, respectively) and increased ventilation and respiratory frequency (by 7%–12% and 5%–16%, respectively) along the studied running intensities. Exercise energy expenditure (approximately 1%–14%) and cost (7.8, 7.4, and 8.0 J·kg−1·m−1 for 30%, 50%, and placebo devices, respectively) were also decreased when using the jaw-protruding splints. The triathlete’s lower limbs’ running pattern changed by wearing the forwarding splints, decreasing the contact time and stride length by approximately 4% and increasing the stride rate by approximately 4%. Conclusions: Wearing a jaw-protruding splint can have a positive biophysical effect on running-performance-related parameters.