This study tested the physiological, thermoregulatory, fatigue and performance responses to mild dehydration (DEH) in ice hockey goaltenders during an on-ice scrimmage, a shootout and two drills. Goaltenders drank no fluid (NF) to induce mild DEH (2.4 ± 0.3% body mass loss) or maintained hydration (<0.5% loss) with water (WAT) or a carbohydrate-electrolyte solution (CES). Mild DEH, compared to WAT and CES, increased on-ice mean and peak core temperature (NF: 39.1 ± 0.1°C, WAT: 38.6 ± 0.1°C, CES: 38.5 ± 0.1°C), heart rate and perceived fatigue and lowered scrimmage save percentage and reaction time. CES was superior to WAT for increasing peak lateral movement power and reducing on-ice perceived exertion. Central and peripheral fatigue, shootout performance and rebound control were similar between conditions. Overall, mild DEH impaired thermoregulation, performance and perceived fatigue during an on-ice scrimmage, shootout and drills compared to ingestion of either fluid.