Calcinosis cutis is characterized by the deposition of calcium salts in the skin and subcutaneous tissue. Calcinosis cutis has different types, but the idiopathic type is considered the rarest type. We present the case of a 10-year-old boy who presented with a skin lesion on his right knee. No other similar nodules were noted elsewhere in the body. The lesion was first noted one year ago, and it slightly increased in size. The lesion was not pruritic and did not ulcerate. No history of previous trauma was provided. On physical examination, a nontender, firm, immobile, reddish, solitary nodule of 2 cm in diameter was observed on the extensor surface of the right knee. The patient underwent complete laboratory investigations that included hematological, biochemical, and immunological parameters, which yielded normal results. Excisional biopsy was performed, and histopathological examination revealed well-circumscribed deposits of basophilic materials in the subcutaneous tissue that is consistent with calcium deposits of calcinosis cutis. Idiopathic calcinosis cutis is a rare condition in children, particularly if it has a unilateral distribution. Proper evaluation should be performed to rule out any associated metabolic or systemic disorders that may alter the management pathway.