We reported the case of an 85-year-old woman who presented with an acutely tender abdomen and underwent abdominal computed tomography (CT) scanning initially reported as showing diverticulitis. After failed conservative management, this patient was taken to theatre, and laparotomy revealed a punctate perforation of the ileum due to a 4 x 3 cm fish fin, which was removed through enterotomy. Retrospective analysis of the initial CT scans showed a foreign body consistent with that removed. This is the first case documented in the literature of perforation of bowel due to fish fin ingestion, as opposed to fish bone. It highlights firstly how common presentations may have an uncommon cause and secondly, how easily a small foreign body can be missed on CT scan. It also highlights and the importance of (i) eliciting a full history of eating habits in those presenting with abdominal pain and (ii) basing intervention on clinical findings. In this case report, we review the wider medical literature on perforation due to foreign body ingestion.