Introduction: Inadequate fluid intake and resulting poor hydration status are associated with worse cognitive functioning, mood, and wellbeing. Gender and racial/ethnic disparities in hydration status have been found among U.S. children/adolescents. Hydration status in the U.S. adult population has not been assessed. We evaluated the prevalence of inadequate hydration among U.S. adults, tested for disparities by race/ethnicity, gender, and age, and evaluated the associations between hydration status and intake of different beverage types. Methods: The sample included 9183 participants aged 20 and older from the National Health and Nutrition Examination Survey 2009-2012. We calculated mean urine osmolality in the population and estimated the proportion of adults, overall and stratified by race/ethnicity, gender, and age, with inadequate hydration using a cutoff of greater than 800 mOsm/kg. Multivariable regression models estimated the associations between demographic variables, intake of beverages, and inadequate hydration, adjusting for confounders. Results: The overall prevalence of inadequate hydration was 28%, with a mean urine osmolality of 611.3 mOsm/kg. Urine osmolality and risk poor hydration status decreased with age. Men (OR=1.75; 95% CI=1.54, 1.98), non-Hispanic blacks (OR=1.65; 95% CI=1.39, 1.96), and Hispanics (OR=1.57; 95% CI=1.37, 1.79) had a significantly higher risk for inadequate hydration than women and non-Hispanic whites, respectively. Consuming water, coffee/tea, diet beverages, and 100% juice were all associated with reduced risk of inadequate hydration, and partially attenuated the racial/ethnic disparities. Conclusions: We confirm similar patterns of disparities by race/ethnicity and gender among adults. Improved intake of healthy beverages may help address inadequate hydration in these populations.