The cellular origin of soft tissue sarcomas (STSs) is not fully understood. The cancer stem cell hypothesis presumes that tumors originate from the malignant transformation of stem cells. As a type of multipotent stem cell, adipose tissue-derived stromal/stem cells (ADSCs), which possess an unexpected degree of plasticity and often reside in other tissues, may represent a potential source of soft tissue sarcoma. To ascertain whether ADSCs are responsible for the formation of STSs, ADSCs from mice were cultured and treated with 3-methycholanthrene to derive transformed cells. These transformed ADSCs were then injected subcutaneously into immunodeficient mice to test their tumorigenic potential. We found that they generated several types of STSs, including synovial sarcoma, malignant fibrous histiocytoma and fibrosarcoma. This is the first study to report that ADSCs may be the potential initiating cells for synovial sarcoma. Our findings indicate that STSs might originate from malignantly transformed ADSCs.