Infantile fibromatosis is a rare mesenchymal disorder characterized by the fibrous proliferation of the skin, bone, muscle, and viscera. The clinical features vary from solitary to multicentric forms with similar pathological features. Although the tumor is histologically benign, it is a highly infiltrating lesion making the prognosis poor for patients with craniofacial involvement affection due to the major risk of nerve vascular and airway compression syndrome. The solitary form of infantile fibromatosis observed in the dermis, subcutis, or fibromatosis tends to occur predominantly in males and typically affects craniofacial deep soft tissues. We present a case of an unusual symptom presentation and a rarely observed location of a solitary fibromatosis form, affecting the muscle of the forearm and infiltrating the bone in a 12-year-old girl. Imaging findings were suggestive of rhabdomyosarcoma, but histopathology set the diagnosis of an infantile fibromatosis. The patient, then, received chemotherapy, and amputation was proposed due to the inextricability of this benign yet aggressive tumor, an option that was refused by her parents. We discuss through this article the clinical, radiological, and pathological features of this benign yet aggressive condition, the potential differential diagnosis, the prognosis, and treatment options substantiated with concrete examples from the literature.