"P. sibiricus" is the largest-known species of "Psittacosaurus". The skull of the type specimen is 20.7 centimetres long (8.25 in), and the femur is 22.3 cm (8.75 in) in length. It is also distinguished by its neck frill, which is longer than any other species, at 15 to 18% of skull length. A very striking feature of "P. sibiricus" is the number of 'horns' around the eyes, with three prominences on each postorbital, and one in front of each eye, on the palpebral bones. Similar horns found on the postorbital of "P. sinensis" are not as pronounced but may be homologous. The jugal has extremely prominent 'horns' and may contact the premaxilla, both features also seen in the possibly related "P. sinensis". There is a flange on the dentary of the lower jaw, similar to "P. mongoliensis", "P. meileyingensis", and "P. sattayaraki". It can be told apart from the other species of "Psittacosaurus" by a combination of 32 anatomical features, including six that are unique to the species. Most of these are skull details, but one unusual feature is the presence of 23 vertebrae between the skull and pelvis, unlike the 21 or 22 in the other species where the vertebrae are known.