First presentation of endometriosis as acute large bowel obstruction is rare. We present the case of a woman in her 40’s with no significant past medical history presenting with acute bowel obstruction. CT imaging revealed an obstructing mass in the sigmoid colon with associated lymphadenopathy, favoured to be a malignant process. She underwent an emergency laparoscopic to open Hartmann’s procedure, where the sigmoid mass was seen causing proximal colonic distention and caecal seromuscular tears. Her case was discussed at the colorectal MDT, where pathology revealed a surprising diagnosis of endometriosis – no primary mucosal lesion was identified. This case highlights endometriosis as an important differential cause of acute bowel obstruction, even in patients with no prior history of endometriosis.