Plesiosaurus philarchus was named by Seeley (1869). Its type specimen is CAMSM J.46913, a partial skeleton, and it is a 3D body fossil. Its type locality is Peterborough area, Porter Collection, which is in a Callovian offshore mudstone in the Oxford Clay Formation of the United Kingdom. It is the type species of Peloneustes.

It was recombined as Thaumatosaurus philarchus by Lydekker (1888); it was recombined as Peloneustes philarchus by Lydekker (1889), Lydekker (1889), Andrews (1913), Swinton (1934), Tarlo (1960), O'Keefe (2001), Araujo et al. (2008), Ketchum and Benson (2011).