Background: Synovial sarcoma is a rare soft-tissue malignant tumor most commonly occurring in the extremities and head and neck region, and rarely occurring in the pericardium. Case presentation: We report a 7-year-old boy was admitted to the hospital with recurrent fever and chest pain over the past four months. A cardiac magnetic resonance imaging (MRI) revealed a tumor beneath the heart in the pericardial, and we surgical resection it completely. The postoperative histopathological examination resulted in a diagnosis of monophasic spindle cell type synovial sarcoma. After two weeks of hospitalization, the patient was discharged. Three months after discharge, the positron emission tomography (PET/CT) scans did not show any signs of recurrence. Conclusion: Pericardial synovial sarcoma is a rare disease that is detected early, and complete resection improves patient survival. We recommend CT be performed in patients with recurrent fever and sizeable pericardial effusion to rule out possible pericardial synovial sarcoma considering the echocardiography limitations.