Synovial sarcoma is rarely detected in infants, with an annual incidence of 0.5 per million. Synovial sarcoma occurs more frequently in adolescents and young adults, with the majority of patients presenting between 15 and 40 years of age. It is extremely rare, however, in pediatric patients under 2 years of age. In the present study we examined a 3-day-old male infant born at 32 weeks who had a mass on his left arm. Synovial sarcoma was identified on histopathological and immunohistochemical analysis of biopsy material acquired from the mass. On whole body magnetic resonance imaging, diffuse metastases were detected in the bilateral lungs in the retroperitoneal zone, in bilateral suprarenal glands, the right liver lobe, the right kidney, and the brain. To our knowledge this is the youngest patient to be diagnosed with synovial sarcoma in the literature. © 2014 Japan Pediatric Society.