Early-onset obesity is known to culminate in type 2 diabetes, arterial hypertension and subsequent cardiovascular disease. The role of sodium (Na+) homeostasis in this process is incompletely understood, yet correlations between Na+ accumulation and hypertension have been observed in adults. We aimed to investigate these associations in adolescents. A cohort of 32 adolescents (13-17 years), comprising 20 obese patients, of whom 11 were hypertensive, as well as 12 age-matched controls, underwent 23Na-MRI of the left lower leg with a standard clinical 3T scanner. Median triceps surae muscle Na+ content in hypertensive obese (11.95 mmol/L [interquartile range 11.62-13.66]) was significantly lower than in normotensive obese (13.63 mmol/L [12.97-17.64]; p = 0.043) or controls (15.37 mmol/L [14.12-16.08]; p = 0.012). No significant differences were found between normotensive obese and controls. Skin Na+ content in hypertensive obese (13.33 mmol/L [11.53-14.22] did not differ to normotensive obese (14.12 mmol/L [13.15-15.83]) or controls (11.48 mmol/L [10.48-12.80]), whereas normotensive obese had higher values compared to controls (p = 0.004). Arterial hypertension in obese adolescents is associated with low muscle Na+ content. These findings suggest an early dysregulation of Na+ homeostasis in cardiometabolic disease. Further research is needed to determine whether this association is causal and how it evolves in the transition to adulthood.