We report on a 50-year-old female patient affected by breast cancer and a tumorous lesion located in the subserosal and intramural part of the uterine fundus. It consisted of glandular and cystic forms lined with epithelium of the tubal type. We describe the clinical, macroscopic, and histological features of this rare non-neoplastic lesion, classified as florid endosalpingiosis (ES) and belonging to the spectrum of Müllerian lesions. The discussion focuses on its morphological characteristics, the differential diagnosis, and the potential reasons leading to its origin. To the best of our knowledge, this is the fifth case of florid ES of uterus reported in the English literature.