Peloneustes is one of the smallest species of pliosaurid known. It may have grown up to 4 metres in length.

It has a long narrow snout with up to 80 teeth in the upper and 80 teeth in the lower jaws.

There have been no stomach contents found associated with a skeleton of Peloneustes philarchus and so its diet is uncertain, however these adaptations suggest it may have eaten fish.