It was named by Kenneth Carpenter based on a complete skull with a mandible, cervical vertebra, and a coracoid.

[2] In naming the specimen, Carpenter noted "Of all known pliosauroids, Plesiopleurodon wellesi most closely resembles Liopleurodon ferox from the Oxfordian of Europe, hence the generic reference.

"[2] It was initially described as a pliosauroid due to it short neck, a common trait of the superfamily (although it is in the order Plesiosauria).

[2] The orbit has a reniform, or kidney-like, shape caused by posterior extension of the maxilla toward the prefrontals.

[8][7] Fischer and colleagues, in 2018, named a new clade, Occultonectia, to include Plesiopleurodon, the 'Richmond pliosaur' (museum number QM F18041), and Sulcusuchus erraini.

[10] This widespread distribution coincides with a global sea level rise and the breakup of Pangaea, allowing for marine corridors to be used for dispersal.

[10] Plesiopleurodon, like all plesiosaurs, are obligatory aquatic animals that rely on paraxial propulsion to swim.

[12] While the Belle Fourche Shale is dated to the Cenomian, most of the fossils discovered there are from the uppermost portion, more than 500 feet above the base of the formation.