This study aims to examine whether mild dehydration affects the cognitive function, mood and subjective feelings among the male football players of Universiti Pendidikan Sultan Idris (UPSI). Fourteen athletes with the mean age of 22±1 years, weight of 65.7 ±7.6 kg and height of 168±6 cm were randomly divided into dehydration and euhydration trials. The baseline data was collected a week before the main trials to record the time taken by the participants to lose 2% of the body weight. Fifteen minutes before conducting the trials, the weight, height, pulse rate and urine samples of the participants were collected and then a warm-up session was conducted for ten minutes. The participants were instructed to exercise until mild dehydration by conducting a slow running on a treadmill within forty-five minutes. The participants from the euhydration trial were given drinks every fifteen minutes throughout the experiment, while the dehydration trial were not provided with drinks. A Visual Analogue Scale (VAS) questionnaire was used to determine the subjective feelings of dizziness and thirst, while the cognitive performance was measured by using the Flanker Test. The Profile of Mood State (POMS) questionaire was utilized to measure the mood changes of the participants. Urine Specific Gravity of the urine samples was measured using Digital Refractometer to determine the hydration status of the participants. The data were analysed using the Paired t-test. There was a significant difference in post-exercise body weight loss for the dehydration trial compared to the euhydration trial. In terms of cognitive performance, there was a significant difference for the component of non-congruent reaction time between the dehydration and euhydration trials. Besides, the findings had shown that there were no significant differences for all six dimensions of mood between the trials and there was a significant different for dizziness level between the trials. In conclusion, this study found that mild dehydration has affected the subjective feeling of dizziness and non-congruent reaction time. Thus, the athletes need to be exposed to the fluid intake strategy and the awareness about hydration status in order to avoid dehydration to optimize physical, mental and sports performance.