Low-grade fibromyxoid sarcoma (LGFMS) is a distinctive fibroblastic neoplasm that is characterized by alternating collagenous and myxoid areas, deceptively bland spindle cell morphology, a whorling architecture, and a t(7;16) translocation involving FUS and CREB3L2. Owing to variable morphology and a lack of discriminatory markers, LGFMS can be difficult to distinguish from benign mesenchymal tumors and other low-grade sarcomas. Gene expression profiling has identified differential upregulation of the mucin 4 (MUC4) gene in LGFMS compared with histologically similar tumors. MUC4 is a transmembrane glycoprotein that functions in cell growth signaling pathways; aberrant MUC4 expression has been reported in various carcinomas. We investigated MUC4 protein expression by immunohistochemistry in LGFMS and in other soft tissue tumors to determine the potential diagnostic use of this novel marker. Whole-tissue sections of 309 tumors were evaluated: 49 LGFMSs (all with FUS gene rearrangement confirmed by fluorescence in situ hybridization), 40 soft tissue perineuriomas, 40 myxofibrosarcomas, 20 cellular myxomas, 20 solitary fibrous tumors, 20 low-grade malignant peripheral nerve sheath tumors, 20 cases of desmoid fibromatosis, 20 neurofibromas, 20 schwannomas, 20 monophasic synovial sarcomas, 20 cases of dermatofibrosarcoma protuberans, 10 myxoid liposarcomas, and 10 extraskeletal myxoid chondrosarcomas. The LGFMS cases included 7 with marked hypercellularity, 4 with prominent hemangiopericytoma-like vessels, 3 with giant collagen rosettes, 3 with epithelioid morphology, 2 with focal nuclear pleomorphism, and 2 with areas of sclerosing epithelioid fibrosarcoma. All 49 LGFMS cases (100%) showed cytoplasmic staining for MUC4, which was usually diffuse and intense. All the other tumor types were negative for MUC4, apart from 6 (30%) monophasic synovial sarcomas. In conclusion, MUC4 is a highly sensitive and quite specific immunohistochemical marker for LGFMS, and can be helpful to distinguish this tumor type from histologic mimics.