Abstract Background More than 75% of children do not meet the water intake recommendations by the Institute of medicine. In addition, more than 80% of children in many European countries do not achieve the water intake recommended by the guidelines of the European food safety authority. Studies have revealed a correlation between hydration status and cognitive achievements in visual attention, immediate memory span, semantic flexibility, and automatic application of arithmetic operations in school children living in hot climates. Aim of the Work Assessment of the hydration status in Egyptian primary school children and determining the relationship between hydration status and cognitive function. Patients and Methods A cross-sectional questionnaire-based study was conducted on 200 children, the age group was from 9 to 13 years old. Data collection included the governmental, private and international schools in Cairo. The questionnaire was fulfilled during school time. A child-friendly “fluids and liquid food” diary was designed to prospectively record the volume of total fluid intake including water, sugar sweetened beverages and fruit juices in the previous day and during school day. Hydration status was assessed by self-reported urine colour using eight-point colour scale. The students were asked to answer four cognitive function tests that were designed to test the cognitive skills which were known to be affected by dehydration (Visual attention, Visual memory, Visual search, Visuomotor performance). Results 54 children (27%) had “Satisfactory knowledge” i.e. the children who met the criteria in the self- answered questionnaire and 146 children (73%) of them had “Unsatisfactory knowledge of proper total fluid intake. 166 children (83%) reported low total fluid intake and 34 children (17%) had adequate total fluid intake (according to the guidelines of the European food safety authority), 118 children (59%) were dehydrated, and 34 children (17%) were severely dehydrated. There was a statistically significant higher total fluid intake in international school regarding the amount of total fluid intake during school time and the amount of total fluid intake per day. In our study, the most consumed fluids after water were sugar sweetened beverages and fruit juices. There was a highly statistically significant positive correlation between total fluid intake and cognitive function regarding visual attention, visual memory, visual search, visuomotor performance and total score of cognitive functions, with p-value (p < 0.001). Conclusion We concluded that there was a significant correlation between school students’ hydration status and total fluid intake and their cognitive functions regarding their Visual attention, Visual memory, Visual search, Visuomotor performance. That calls for the importance of corroboration of our study results with studies that further explore students’ awareness and perception about their daily need of water and benefits of proper hydration.