Obesity has been found to relate to urine-assessed hypohydration risk at the population level, but studies have often failed to account for lean mass, which influences urine solute load. Adults (n = 8,699) from the National Health and Nutrition Examination Survey were categorized as hypohydrated or not using five urine-based methods. Hypohydration rates varied widely between the methodologies (12.9-37.9%). In comparison to adults with a body mass index (BMI) <25 kg/m[superscript 2], those with a BMI =30 kg/m[superscript 2] had higher odds of hypohydration (odds ratio [OR] = 1.59; 95%CI = 1.30-1.95) when using urine osmolality >850 mOsm/kg. Further, relative to adults with low estimated lean mass, those in the highest tertile had even higher odds of hypohydration (OR = 2.47; 95%CI = 1.96-3.11) when using urine osmolality >850 mOsm/kg. Urine osmolality and creatinine were more strongly associated with estimated lean mass than BMI. Links between obesity and urine osmolality at the population level are partly driven by variations in lean mass.