Barley, OR, Iredale, F, Chapman, DW, Hopper, A, and Abbiss, C. Repeat effort performance is reduced 24 hours after acute dehydration in mixed martial arts athletes. J Strength Cond Res 32(9): 2555-2561, 2018-This study sought to determine the influence of acute dehydration on physical performance and physiology in mixed martial arts (MMA). Mixed martial arts athletes (n = 14; age: 23 ± 4 years) completed in a randomized counterbalanced order a dehydration protocol, (DHY: 3-hour 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 hours and 24 hours after the intervention, whereas urine and blood samples were collected before, 20 minutes, 3 hours, and 24 hours after the intervention. Body mass was reduced (4.8 ± 0.8%) after DHY (p < 0.001) and remained lower than CONT at 3 hours and 24 hours after DHY (p = 0.003 and p = 0.024, respectively). Compared with CONT, average sled push times were slower 3 hours and 24 hours after DHY (19 ± 15%; p = 0.001; g = 1.229 and 14 ± 15%; p = 0.012; g = 0.671, respectively). When compared with the CONT, handgrip was weaker 3 hours after DHY (53 ± 8 and 51 ± 8 kg; p = 0.044, g = 0.243, respectively) and medicine ball chest throw distances were shorter 24 hours after 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 minutes (p = 0.035) and 24 hours (p = 0.035) after DHY. Acute dehydration of 4.8% body mass results in reduced physical performance 3 and 24 hours after DHY. There is need for caution when athletes use dehydration for weight loss 24 hours before competition.