Klinefelter syndrome (KS) is the most common chromosomal disorder in men characterized by clinical features of hypogonadism and infertility. About 90% of cases have classically 47, XXY karyotype and the remaining have additional X or Y chromosomes, high grade aneuploidies or X chromosome structural abnormalities. Portal vein aneurysms are very rare. Reported cases are increasing due to use of modern imaging techniques in clinic practise. Here we report a 19-year-old man with KS who was admitted with complaints of abdominal pain, nausea and vomiting. Further investigations revealed 23 mm anechoic, saccular expansion in the left branch of the portal vein. It is well known that KS is associated with venous thromboembolic diseases including portal venous thrombosis, but association with portal vein aneurysm has not been previously reported.