.Little data exists regarding nutritional knowledge and hydration in adolescent swimmers. The aim of this study was to assess the level of nutrition and hydration knowledge and to describe the fluid balance in adolescent swimmers during training. A study was carried out with a cross-sectional descriptive part and a longitudinal part with repeated measurements over five swimming sessions. Eighty-six adolescent swimmers completed a questionnaire to assess their sport nutrition and hydration knowledge. Fluid balance and urine hydration markers were studied during training. Swimmers showed a limited nutrition knowledge (33.26 % ± SD 12.59) and meagre hydration knowledge (28.61 % ± SD 28.59). Females showed lower scores than male swimmers in nutrition and hydration knowledge. Based on urine specific gravity swimmers started the training close to the euhydrated threshold (1.019 g/mL ± SD 0.008). Although urine specific gravity and urine colour were reduced after the training there were minimal changes in body mass (-0.12 Kg ± SD 0.31). Sweat loss (2.67 g/min ± SD 3.23) and the net changes in the fluid balance (-0.22 % ± SD 0.59) were low. The poor knowledge in nutrition and hydration encountered in the swimmers can justify the development of a strategy to incorporate nutritional education programmes for this group. Body water deficit from swimming activity seems to be easily replaced with the water intake to maintain hydration. After the training the urine of swimmers was diluted regardless of their water intake. Dilution of urine did not reflect real hydration state in swimming.