A 6-year-old neutered female domestic shorthair cat weighing 3.1 kg was referred to the clinic owing to a 2 day history of anorexia. On physical examination, a pectus excavatum (PE) was apparent, and a cranial abdominal mass was palpated. Serum biochemical analysis showed severely elevated liver enzymes. On radiography, the PE was graded as moderate, and a 5 × 3 cm soft tissue density mass was detected in the cranial abdomen. Using duplex ultrasonography, the mass was identified as an enlarged liver lobe with vascular thrombosis. The diagnosis of liver lobe torsion (LLT) was confirmed surgically, and the affected left lateral lobe was resected. On follow-up, performed 1 and 6 months postoperatively, no further abnormalities were found on clinical examination or serum biochemical analysis. At follow-up examination, the PE did not appear to cause any additional clinical signs, indicating that treatment of moderate PE in older animals is not obligatory. This is the first reported case of LLT with concurrent PE, and the second feline case of LLT treated surgically. Animals with anatomical malformation of the diaphragm caused by PE may be at higher risk of developing LLT. Early diagnosis and surgical intervention of LLT can produce a good clinical outcome without further repercussions.