Congenital longitudinal melanonychia is rarely seen in Caucasians and poses a diagnostic dilemma. It is characterized by the presence of hyperpigmented brown or black streaks visible in the nail plate. We present the case of 20-month-old boy with congenital longitudinal melanonychia, the diagnostic procedure, and management. Dermatoscopy of the nail revealed dark brown linear hyperpigmented lines in the nail plate with an irregular color. The diagnosis of a congenital nail matrix nevus was established on clinical grounds. A wait-and-see policy was taken. A biopsy was not performed because the cause of longitudinal melanonychia is essentially benign, especially in the congenital form. Subungual melanomas are very rare and mostly appear in middle-aged patients. Clinical, dermatoscopic, and histopathological guidelines that could directly distinguish between a benign nail matrix nevus and malignant subungual melanoma in the pediatric population have not been confirmed yet. The diagnostic dilemma therefore remains open.