Hypohydration is generally considered to have a negative effect on cognitive function, despite several studies reporting comparable findings between hydration states. Recommendations to avoid moderate dehydration (≥ 2% loss in body mass) are commonly made to athletes, on the provision that this deficit may impair optimal cognitive performance. To determine whether cognitive function is impaired by hypohydration, and investigate the existence of the proposed critical water deficit of ≥2% loss in body mass purported to diminish cognitive performance, we conducted a systematic search of the literature and examined appropriate studies by meta-analysis. Overall, cognitive performance was not found to be impaired by hypohydration (g = −0.177; 95% CI = −0.532-0.179; P =.331). Nor were the underlying cognitive domains (complex attention, executive function, learning and memory) impaired (all P >.236), independent of the incurred fluid loss (less than or >2% loss in body mass), although results were not always homogenous (I 2 ranging between 0% and 93%). Collectively, these results suggest that hypohydration may not compromise cognitive function, nor any of the investigated subdomains to a greater extent than if euhydration had been maintained. Furthermore, recommendations to avoid moderate hypohydration on the basis of maintaining optimal cognitive function are not substantiated by this meta-analysis.