Thalassomedon (from Greek, thalassa, "sea" and Greek, medon, "lord" or "ruler", meaning "sea lord") is a genus of plesiosaur, named by Welles in 1943.

[3] The neck is also very long; it comprises 62 vertebrae[4] and is about 5.9 metres (19 ft) - over half of the total length.

Stones have been found in its stomach area leading some to theorize that they were used for ballast or digestion.

This genus of plesiosaur lived in North America, approximately 95 million years ago - this places it during the Cenomanian stage.

Its closest relative is Elasmosaurus, and both belong to the family Elasmosauridae.