A 10-year-old pet chinchilla (Chinchilla lanigera) was referred for ultrasound investigation of a thoracic mass. The mass was initially believed to be a pulmonary abscess or tumor based on radiographs and ultrasound. Cytological examination suggested the presence of a gastrointestinal structure in the thorax, and necropsy revealed a true diaphragmatic hernia subdividing the stomach into thoracic and abdominal portions.