This school-based, cross-sectional study aimed to evaluate whether hypohydration is related to functional constipation and physical activity in school-aged children. The study included 452 students aged 6-12 years. Hypohydration, defined as urinary osmolality >800 mOsm/kg, was more prevalent ( P = 0.002) in boys (72.1%) than in girls (57.5%). The difference in the prevalence of functional constipation according to sex (20.1% in boys and 23.8% in girls) was not statistically significant ( P = 0.81). In bivariate analysis, functional constipation was associated with hypohydration in girls (odds ratio = 1.93, 95% confidence interval: 1.07-3.49), and multiple logistic regression did not reach statistical significance ( P = 0.082). Low proportions of active commuting to school in both sexes were associated with hypohydration. However, there were no associations between functional constipation, active commuting to school, and physical activity scores. In conclusion, multiple logistic regression did not demonstrate an association between hypohydration and functional constipation in school-aged children.