The Bosniak classification of renal cysts aims to provide a probabilistic risk assessment indicating the likelihood of malignancy from imaging findings. Originally designed to classify adult renal cysts based on computed tomography findings, the Bosniak classification has been extended to pediatric patients, with some adjustments made with the aim of accommodating magnetic resonance imaging (MRI) and ultrasonography (US). Bosniak IV lesions are rare in adolescents, indicating localized renal cell carcinoma and requiring surgical intervention. In contrast, Bosniak III lesions can be treated conservatively, although there is a lack of specific guidelines on their management. We present a case of a 14-year-old boy with a Bosniak III lesion, which was incidentally detected during the US evaluation of a left varicocele. After a 12-month follow-up, MRI revealed progression to a Bosniak IV cyst. Robot-assisted tumor enucleation was performed with selective artery clamping when the patient was 15. Histopathology showed tubulocystic renal cell carcinoma without adverse features. Immunocytochemistry supported a favorable prognosis of this rare tumor (<1% of renal tumor), thus obviating the need for adjuvant treatment. At the 18-month follow-up, no recurrence or distant metastasis were observed. This case highlights the importance of an aggressive treatment in persistent Bosniak III and Bosniak IV renal cysts in children and adolescents and the necessity to offer a nephron-sparing surgery.