Background: Pediatric hypertension is an increasingly recognized health concern and is commonly influenced by modifiable factors such as dietary sodium intake and obesity and non-modifiable factors like family history of hypertension. Urinary sodium excretion provides an objective surrogate marker of sodium consumption and may be associated with blood pressure severity. This study aimed to evaluate urinary sodium excretion in children with primary hypertension (PH) and to test the hypothesis that higher sodium excretion is associated with less favorable clinical, biochemical, and blood pressure parameters. Methods: This retrospective, cross-sectional, single-center study analyzed data from 369 hypertensive patients and 59 healthy children. Patients with a confirmed diagnosis of PH and ambulatory blood pressure monitoring results were included in the study group. Clinical, anthropometric, laboratory, echocardiographic, and blood pressure data were examined, and sodium excretion was evaluated using both the spot urine sodium-to-creatinine ratio and 24-h urinary sodium per kilogram of body weight. Results: Children with hypertension exhibited higher urinary sodium excretion compared to the control group. Sodium excretion of the hypertensive group, measured using the sodium/creatinine ratio and 24 h urinary sodium excretion per kilogram, was positively correlated with 25-hydroxyvitamin D, the urinary potassium/creatinine ratio, and the urinary uric acid/creatinine ratio. Moreover, negative correlations were observed for both parameters with age, body weight, serum uric acid, and left ventricular mass. In the multivariate analysis, weighted Z-score (beta = −0.393), age (beta = −0.293), 25-OHD (beta = 0.182), and arterial hypertension in the father (beta = 0.166) predicted 24 h urinary sodium excretion. Children with excessive sodium excretion had a significantly higher systolic blood pressure load over 24 h. Conclusions: Urinary sodium excretion is elevated in children with PH. Children with a lower weight for their age, who are younger, and who have a father with arterial hypertension might be at higher risk of excessive urine excretion. Our findings underscore the clinical importance of dietary sodium reduction as a non-pharmacological therapeutic target, especially in these patient populations. Prospective studies are needed to evaluate its impact on long-term cardiovascular outcomes in this population.