Soft tissue sarcomas in the first year of life are rare, and the most common sarcomas in infancy are embryonal rhabdomyosarcoma, Ewing sarcoma/primitive neuroectodermal tumor, congenital infantile fibrosarcoma, and primitive sarcomas such as undifferentiated sarcoma. In this study, we report 6 cases of a primitive myxoid mesenchymal tumor of infancy (PMMTI), which previously may have been included under the diagnostic categories of congenital-infantile fibrosarcoma or infantile fibromatosis. PMMTI occurred in 6 infants, 3 of whom had a congenital presentation of a soft tissue mass. All patients were otherwise healthy. The tumors occurred on the trunk, extremities, and head and neck. Grossly, the tumors were nonencapsulated and had a multinodular appearance with focal infiltrative growth, a white fleshy cut surface, and a tumor diameter ranging from 2 to 15 cm. Histologically, a diffuse growth of primitive spindle, polygonal, and round cells occurred in a myxoid background. The tumor cells were arranged in a vaguely nodular pattern with peripheral collagenized stroma, higher cellularity at the periphery, and a delicate vascular network in the background. Immunohistochemically, the tumors displayed diffuse reactivity for vimentin and no reactivity for smooth muscle actin, muscle specific actin, desmin, S-100 protein, or myogenin. Electron microscopy documented a poorly differentiated fibroblastic proliferation. Four cases tested negative for the ETV6-NTRK3 gene fusion by RT-PCR. One tumor had a complex karyotypic abnormality with rearrangements involving chromosomes Y, 9, and 3. Three patients had recurrences or metastasis treated with a combination of surgery and chemotherapy. One patient is alive with persistent locally aggressive disease, 2 are alive with no evidence of recurrence, 1 had a recurrence treated surgically without further follow-up information, 1 patient died with persistent tumor and sepsis 6 weeks after diagnosis, and 1 patient was lost to follow-up. The morphologic appearance combined with the ultrastructural features and absence of the typical gene rearrangement of congenital-infantile fibrosarcoma are unique, and we propose that PMMTI represents a new category of pediatric fibroblastic-myofibroblastic tumor.