This study examined the acute physiological and cognitive effects of pre-competition rapid weight loss and gain (weight cutting) on mixed martial arts (MMA) athletes. 60 (8 female; 52 male) licensed amateur and professional MMA athletes participated in the study. Measurements were collected at three time points prior to a competition: 10–14 days (Time-1; n=50), 24 hours (Time-2; n=40) and 1–3 hours (Time-3; n=26). Measurements included body mass (kg), King-Devick Test (KD), Sit-to-Stand Heart Rate Test (SSHR), Vertical Jump Test (VJ), grip strength (kg) and urine specific gravity (USG) (mmol). Also, relative change [((Time-1 – Time-2)/Time-1) × 100] in both body mass and USG were compared to other dependent variables. The following variables [mean(SD)] at Time-2 were different from Time-1 before returning to near baseline at Time-3: body mass [74.44 (13.11) vs 70.77 (12.26) vs 74.07 (13.33) kg; F (1,1.24)=50.72, p=0.00], left grip strength [111 (27) vs 98 (25) vs 108 (27) kg; F (1,2)=23.38, p=0.00], right grip strength [112 (25) vs 100 (23) vs 110 (24) kg; F (1,2)=17.91, p=0.00], USG [1.008 (0.003) vs 1.032 (0.004) vs 1.007 (1.003) mmol; F (1,2)=299, p=0.00] and KD sum time (KDst) adjusted for errors [41.39 (4.78) vs 42.44 (5.92) vs 38.58 (4.58) s; F (1,1.41)=8, p=0.00]. Relative change in USG was significantly correlated with relative change in body mass [rs (30)=−0.405; p=0.03]; and left leg VJ force at Time-2 [rs (18)=−0.71; p=0.00] and Time-3 [rs (11)=−0.64; p=0.04]. Relative change in body mass was significantly correlated with KDst at Time-2 [rs (30)=−0.57; p=0.00] and Time-3 [rs (21)=−0.49; p=0.00]; and SSHR maximum at Time-2 [rs (30)=0.41; p=0.03] and Time-3 [rs (21)=0.49; p=0.03]. Findings indicate that a significant amount of weight loss while weight cutting was due to fluid loss that was sufficient at Time-2 to impair upper body strength, lower body power, SSHR response and cognitive function (higher KDst and errors). In athletes who experienced the most Time-2 fluid loss, the return of body mass and USG to near baseline levels at Time-3 did not fully reverse the effects on SSHR, lower body power and some cognitive functions (KDst). It was concluded that weight cutting causes a substantial disruption to physiology that may impact health and performance.