An unusual case of ovarian adenosarcoma arising from a smooth-walled serous cystadenoma is described. The ovary was replaced by a multiloculated, fluid-filled cyst without any solid or papillary areas. The malignant component was underdiagnosed during frozen section examination as benign cystadenoma because of the deceptively benign gross appearance of the tumor. On the permanent sections, a phyllodes-like pattern of stromal proliferation and periglandular condensation of atypical stromal cells with a mitotic count of 3 per 10 high-power fields was more apparent and led to the diagnosis of adenosarcoma. The malignant component could not be distinguished from the benign component using immunohistochemical analysis. To our knowledge, this is the first reported case of an adenosarcoma arising from a grossly benign cystadenoma and the third case in the literature of an adenosarcoma associated with a cystadeno(fibro)ma. This case also shows the challenges in differentiating adenosarcoma from a benign counterpart on both frozen and permanent sections.