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

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Thalassomedon Temporal range: Late Cretaceous, 95 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Mounted cast of the type specimen, American Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Family:	†Elasmosauridae
Genus:	†Thalassomedon Welles, 1943
Species:	†T. haningtoni
Binomial name
†Thalassomedon haningtoni Welles, 1943
Synonyms
Alzadasaurus riggsi Welles, 1943

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Thalassomedon is among the largest elasmosaurids, with the holotype measuring 10.86 metres (35.6 ft) long and weighing more than 4.44 metric tons (4.89 short tons).^[1][2] There is a larger skull, however, suggesting a much larger animal, potentially up to 11.6 metres (38 ft).^[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. The skull is 47 centimetres (19 in; 1.54 ft) long, with 5 centimetres (2.0 in) long teeth. The flippers were about 1.5–2 metres (4.9–6.6 ft) long. Stones have been found in its stomach area leading some to theorize that they were used for ballast or digestion. If the latter, stomach action would cause the stones to help grind ingested food.

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. There are six specimens of varying states of preservation on display at various museums in the United States.

• List of plesiosaur genera
• Timeline of plesiosaur research