A 3-month-old female infant was admitted for incessant crying for 3 days. Examination revealed a rapidly growing massive retroperitoneal mass that was difficult to resect. Needle biopsy confirmed infantile fibrosarcoma. Initial chemotherapy with the VAC regimen (vincristine, actinomycin D, and cyclophosphamide) was administered, but the response was poor. The common fusion in infantile fibrosarcoma is ETV6-NTRK3, and next-generation sequencing detected an RBPMS-NTRK3 gene fusion in this patient. To our knowledge, this is the first reported case of infantile fibrosarcoma with RBPMS-NTRK3 fusion in China. Treatment with larotrectinib resulted in marked tumor shrinkage.