This study sought to determine the influence of acute dehydration on physical performance and physiology in Mixed Martial Arts (MMA). MMA athletes (n=14; age: 23±4 years), completed in a randomised counterbalanced order a dehydration protocol, (DHY: 3 h cycling at 60 W in 40°C to induce 5% dehydration) or thermoneutral control (25°C: CONT) exercise, followed by ad libitum fluid/food intake. Performance testing (a repeat sled push test, medicine ball chest throw and vertical jump) was completed 3 and 24 h following the intervention, while urine and blood samples were collected before, 20 min, 3 and 24 h following the intervention. Body mass was reduced (4.8±0.8%) following DHY (p<0.001) and remained lower than CONT at 3 and 24 h post (p=0.003 and p=0.024, respectively). Compared to CONT average sled push times were slower 3 and 24 h following DHY (19±15%; p=0.001; g=1.229 and 14±15%; p=0.012; g=0.671, respectively). When compared to the CONT hand grip was weaker 3 h following DHY (53±8 and 51±8 kg; p=0.044, g=0.243 respectively) and medicine ball chest throw distances were shorter 24 h following DHY (474±52 and 449±44 cm; p=0.016, g=0.253 respectively). No significant differences were observed in vertical jump (p=0.467). Urine specific gravity was higher than CONT 20 min (p=0.035) and 24 h (p=0.035) following DHY. Acute dehydration of 4.8% body mass results in reduced physical performance 3 and 24 h following. There is need for caution when athletes use dehydration for weight loss 24 h prior to competition.