Identifying mild dehydration (≤2% of body mass) is important to prevent the negative effects of more severe dehydration on human health and performance. It is unknown whether a single hydration marker can identify both mild intracellular and extracellular dehydration with adequate diagnostic accuracy (≥0.7 receiver operating characteristic-area under the curve (ROC-AUC)). Thus, in 15 young healthy men, we determined the diagnostic accuracy of 15 hydration markers after three randomized 48-h trials; euhydration (EU, water 36 ml·kg·d-1), intracellular dehydration caused by exercise and 48 h of fluid restriction (ID, water 2 ml·kg·d-1), and extracellular dehydration caused by a 4 h diuretic-induced diuresis, begun at 44 h (ED, Furosemide 0.65 mg·kg-1). Body mass was maintained on EU and dehydration was mild on ID and ED (1.9 (0.5)% and 2.0 (0.3)% of body mass, respectively). Urine color, urine specific gravity, plasma osmolality, saliva flow rate, saliva osmolality, heart rate variability and dry mouth identified ID (ROC-AUC; range 0.70-0.99) and postural heart rate change identified ED (ROC-AUC 0.82). Thirst 0-9 scale (ROC-AUC 0.97 and 0.78 for ID and ED) and urine osmolality (ROC-AUC 0.99 and 0.81 for ID and ED) identified both dehydration types. However, only thirst 0-9 scale had a common dehydration threshold (≥4; sensitivity and specificity of 100%, 87% and 71%, 87% for ID and ED). In conclusion, using a common dehydration threshold ≥4, the thirst 0-9 scale identified mild intracellular and extracellular dehydration with adequate diagnostic accuracy. In young healthy adults’ thirst 0-9 scale is a valid and practical dehydration-screening tool.