The purpose of this study was to characterize the magnitude of acute weight gain (AWG) and dehydration in mixed martial arts (MMA) fighters prior to competition. Urinary measures of hydration status and body mass were determined ∼24 h prior and then again ∼2 h prior to competition in 40 MMA fighters (Mean ± SE, age: 25.2 ± 0.65 yr, height: 1.77 ± 0.01 m, body mass: 75.8 ± 1.5 kg). AWG was defined as the amount of body weight the fighters gained in the ∼22 h period between the official weigh-in and the actual competition. On average, the MMA fighters gained 3.40 ± 2.2 kg or 4.4% of their body weight in the ∼22 h period prior to competition. Urine specific gravity significantly decreased (P < 0.001) from 1.028 ± 0.001 to 1.020 ± 0.001 during the ∼22 h rehydration period. Results demonstrated that 39% of the MMA fighters presented with a Usg of greater than 1.021 immediately prior to competition indicating significant or serious dehydration. MMA fighters undergo significant dehydration and fluctuations in body mass (4.4% avg.) in the 24 h period prior to competition. Urinary measures of hydration status indicate that a significant proportion of MMA fighters are not successfully rehydrating prior to competition and subsequently are competing in a dehydrated state. Weight management guidelines to prevent acute dehydration in MMA fighters are warranted to prevent unnecessary adverse health events secondary to dehydration.