Mass-like cystic endosalpingiosis is very rare. The author reports such a lesion with histologic features of endosalpingiosis of the uterus with florid epithelial proliferation in a 51-year-old female who complained of lower abdominal pain. Preoperatively, it was considered to be left-sided ovarian cancer. Intraoperatively, a subserosal, sessile polypoid mass with multiple cysts in the uterine fundus measuring 12×6.5×5.5 cm was found. Histologically, it was composed of multiple cysts lined by benign-appearing tubal epithelium and bland smooth muscular and myofibromatous stroma. Part of the epithelium exhibited marked papillarity and tufting. Features of conventional endometriosis were present focally. Pathologists and clinicians should be aware of the existence of this type of non-neoplastic lesion, mass-like cystic endosalpingiosis, and should avoid overdiagnosis and overtreatment.