Sarcomas are a heterogeneous group of malignant tumors that come from mesenchymal tissues. Undifferentiated sarcoma represents approximately 20% of soft tissue sarcomas. This entity represents approximately 20% of soft tissue sarcomas. These tumors are subdivided according to their appearance. Morphological in 4 subtypes: pleomorphic cells, fusiform cells, round cells, epithelioids. We report the case of a 72-year-old woman operated for a complicated adnexal tumor, but it turned out that it was sarcoma undifferentiated (unclassified) fusiform cell type of mesentery. It is a rare and a latent tumor. Its diagnosis is histological. Its treatment consists on surgical wide excision if possible. This type of sarcoma has a poor prognosis considering the limited benefits of radio-chemotherapy. Undifferentiated sarcoma type fusiform cells of the mesentery is an exceptional entity. Its diagnosis is difficult. Its treatment is to discuss case by case, surgery is the best option if it is possible. The prognosis is bad. This entity remains to be studied.