Sclerosing epithelioid fibrosarcoma is a rare variant of fibrosarcomas, which was recently identified as a separate entity due to specific histologic and immunohistochemistry features and its poor prognosis. We report a case of sclerosing epithelioid fibrosarcoma of soft tissues, which developed in a 37-year-old woman who presented a tumor involving the posteromedial aspect of the left knee and which progressed in size for one year. Imaging revealed a well-delimited tumor process measuring 8 cm in its largest diameter and situated in the medial compartment of the left knee. Histology of the tumorectomy specimen and the immunohistochemistry study led to the diagnosis of sclerosing epithelioid fibrosarcoma of soft tissues. This new case illustrates the characteristic features of this tumor and recalls the difficult pathological diagnosis.