Synovial sarcoma is a rare malignant soft-tissue tumor that most commonly occurs in the extremities of young adults. Primary pericardial synovial sarcoma is extremely rare. We report the case of a 37 year old male patient who presented with intermittent fever, nocturnal sweating and asthenia. Chest X-ray revealed an enlarged cardiac silhouette. Echocardiography identified pericardial effusion and a mass compressing the right ventricle. After percutaneous drainage of the effusion, the mass was not visible and deemed to have been septations of the effusion. Chest computed tomography (CT) did not show the mass visible on the X-ray. At one month follow-up, the pericardial mass was again visible on echocardiography and confirmed by magnetic resonance imaging (MRI). CT-guided biopsy showed malignant mesenchymal cells. Complete resection was attempted, but not possible due to diffuse infiltration of the epicardium. Histological examination of the resected tissue revealed an undifferentiated primary pericardial synovial sarcoma. The patient refused adjuvant treatment and died 15 days later. Surgical resection is considered the cornerstone of treatment of this rare type of malignant tumor of the young; our patient presented with a diffusely infiltrating tumor which could not be resected and required reoperation for tamponade and left pericardectomy. We question whether the choice to attempt resection was beneficial.