Williston[1][2] Styxosaurus is named for the mythological River Styx (Στυξ), which separated the Greek underworld from the world of the living.

The specimen was originally described and named Alzadasaurus pembertoni by Welles and Bump (1949) and remained so until it was synonymized with S. snowii by Carpenter.

[6] In 2023, another species of Styxosaurus, S. rezaci, was named by Smith and O'Keefe, based on a specimen from the Cenomanian of Nebraska previously thought to belong to Thalassomedon.

[9] Styxosaurus snowii is from a group called elasmosaurs, and is closely related to Elasmosaurus platyurus, which was found in Kansas, USA, in 1867.

[1] The specimen included a complete skull and more than 20 cervical vertebrae ( KUVP 1301) that were found near Hell Creek in western Kansas by Judge E.P.

[4][12] The following cladogram shows the placement of Styxosaurus within Elasmosauridae following an analysis by Otero, 2016:[13] Eromangasaurus carinognathus Callawayasaurus colombiensis Libonectes morgani Tuarangisaurus keyesi Thalassomedon haningtoni CM Zfr 115 Hydrotherosaurus alexandrae Futabasaurus suzukii Kaiwhekea katiki Alexandronectes zealandiensis Morturneria seymourensis Aristonectes parvidens Aristonectes quiriquinensis Terminonatator pointeixensis Elasmosaurus platyurus Albertonectes vanderveldei Styxosaurus sp.