Dent’s disease is a rare, genetically determined tubulopathy associated with a mutation on the X chromosome, characterized by the occurrence of low molecular weight proteinuria (LMWP), hypercalciuria, and at least one of the following symptoms: nephrocalcinosis, nephrolithiasis, hematuria, hypophosphatemia or renal failure. In the early stages of the disease, the only symptom may be proteinuria. We present a case of a 5-year-old boy admitted to the clinic due to proteinuria, which was detected during a follow-up examination after an upper-airways infection. Further diagnostics also showed signs of hypercalciuria. The family history revealed the death of the mother’s relative at the age of 32, probably due to renal failure, and kidney diseases in relatives on the father’s side. Due to persistent nephrotic-range proteinuria, a kidney biopsy was performed, which revealed focal segmental glomerulosclerosis (FSGS). Genetic testing was performed due to the increased presence of low-molecular-weight proteins and hypercalciuria in the urine, which confirmed a de novo mutation in the CLCN5 gene, indicating Dent’s disease. Treatment included the use of an angiotensin-converting enzyme inhibitor and a thiazide diuretic. The above case confirms that the rare occurrence of Dent’s disease presents diagnostic difficulties in patients with proteinuria. However, proper diagnosis, including the evaluation of low-molecular-weight proteins, calcium levels in urine, and genetic testing, can avoid immunosuppressive treatment, which is ineffective and associated with adverse side effects.