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Dehydration (DEH) is believed to impair cognitive performance but which domains are affected and at what magnitude of body mass loss (BML) remains unclear.To conduct systematic literature review and meta-analysis to determine the effect size (ES) of DEH on cognitive performance and influence of experimental design factors (e.g., DEH > 2% BML).Thirty-three studies were identified, providing 280 ES estimates from 413 subjects with DEH ranging from 1% to 6% BML. Outcome variables (accuracy, reaction time), cognitive domains, and methods to induce DEH varied. Effect sizes were calculated using standardized mean differences and multivariate meta-analysis.Impairment of cognitive performance (all domains/outcomes) with DEH was small but significant (ES = −0.21; 95% confidence interval [CI]: −0.31 to −0.11; P < 0.0001) with significant heterogeneity (Q(279) = 696.0, P < 0.0001; I2 = 37.6%). Tasks of executive function (ES = −0.24; 95% CI: −0.37 to −0.12), attention (ES = −0.52; 95% CI: −0.66 to −0.37), and motor coordination (ES = −0.40 to 95% CI: −0.63 to −0.17) were significantly impaired (P ≤ 0.01) after DEH, and attention/motor coordination was different (P < 0.001) from reaction time specific tasks (ES = −0.10; 95% CI: −0.23 to 0.02). Body mass loss was associated with the ES for cognitive impairment (P = 0.04); consequently, impairment was greater (P = 0.04) for studies reporting >2% BML (ES = −0.28; 95% CI: −0.41 to −0.16) compared with ≤2%; (ES = −0.14; 95% CI: −0.27 to 0.00).Despite variability among studies, DEH impairs cognitive performance, particularly for tasks involving attention, executive function, and motor coordination when water deficits exceed 2% BML.