Low-grade fibromyxoid sarcoma (LGFMS) is a rare neoplasm commonly affecting young adults and typically arising in the somatic soft tissue of the proximal extremities. Its occurrence within the thoracic cavity is exceedingly rare. We report an LGFMS arising from the epicardial surface of the right side of the heart in a 44-year-old woman. Diagnosis was aided by a strikingly characteristic light microscopic appearance of a bland spindle cell sarcoma containing numerous so-called giant collagen rosettes and supported by immunohistology, evidence of FUS translocation by fluorescence in situ hybridization, and electron microscopy demonstrating a fibroblastic phenotype. Aspiration cytology showed a nonspecific bland spindle cell lesion. Review of the literature uncovered 5 previously reported examples of intrathoracic LGFMS. Low-grade fibromyxoid sarcoma is probably an underrecognized intrathoracic neoplasm and should be considered in the differential diagnosis of spindle cell neoplasms of the mediastinum, pleura, heart, and lungs.