Background: Excessive sodium intake is excreted through urine and could affect hydration status. This study aims to describe hydration status in adolescents and to assess the association between hydration status by free water reserve (FWR) and urinary sodium excretion. Methods: Two hundred participants (118 girls), aged 13-18 years completed the study. Median urinary sodium excretion was measured in one 24-hour collection and was used as a proxy for sodium intake. FWR (measured urine volume minus the obligatory urine volume) was used for characterization of hydration status, and linear regression models were used to evaluate the association between urinary sodium excretion and FWR, adjusted for water and energy intake in boys and carbohydrate, fiber, and water intake in girls. Results: The participants median urinary sodium excretion was 3645.5 mg/d for boys and 2702.5 mg/d for girls (p < 0.001). Median FWR was positive in both sex groups; however, 40.2 % of boys and 31.4 % of girls (p = 0.195) were at risk of hypo-hydration status. Linear regression models showed that urinary sodium excretion was a significant predictor of FWR for both sexes (Crude Model β = 0.114, p = 0.003 for boys and β = 0.160, p < 0.001 for girls; Adjusted Model β = 0.120, p = 0.002 for boys and β = 0.142, p < 0.001 for girls). Conclusions: Nearly one third of subjects were at risk of hypo-hydration. Higher sodium excretion was associated with a better hydration status obtained by FWR. However, as the majority of subjects consume sodium above recommendations, preventive measures to promote better hydration status should focus in increasing the level of total water intake.