Arias-Stella reaction is a hormone-induced atypical endometrial change that may involve endometrial, extraendometrial, or endometriotic sites. Establishing a correct diagnosis of Arias-Stella reaction may sometimes be problematic when seen outside of the uterine corpus, particularly in a limited biopsy sample when the pathologist is not aware of the clinical history. We report an Arias-Stella reaction in bladder endometriosis, which presented as a bladder mass in a 25-year-old pregnant woman. Clinical information of intrauterine pregnancy (week 16) was not initially provided when the biopsy was performed. We also discuss endometriosis, endocervicosis, and müllerianosis of the bladder, which are benign glandular lesions that may mimic tumors within the bladder wall.