Round ligament tumours represent a rare entity that can present similarly to an incarcerated hernia. Basic understanding and appropriate preoperative management is imperative in order to differentiate between the two diagnoses. Leiomyoma is the most common type of round ligament tumour. It is associated with oestrogen exposure and is more common in the presence of uterine leiomyomas. Here we discuss a 68-year-old woman who presented with a palpable left inguinal mass that progressively grew in size, associated with pelvic pressure and discomfort. On surgical resection, the mass was found to be derived from the round ligament at the entrance of the external inguinal ring. Pathology confirmed a round ligament leiomyoma, measuring 25×9×8.5 cm. This case is the largest round ligament leiomyoma recorded to date and the first to exhibit carneous degeneration. A review of the current literature is also provided.