Thalassochelydia is a clade of extinct marine turtles from the Late Jurassic and earliest Cretaceous of Europe and South America.

The group is defined as including Eurysternum, Plesiochelys and Thalassemys to the exclusion of Pelomedusa, Testudo and Protostega.

[1] The monophyly of Thalassochelydia was further supported by a later analysis by Serjocha Evers and Roger Benson in 2019, where the group, represented by Solnhofia and Plesiochelyidae was sister taxon to Sandownidae within the clade Angolachelonia, which, unlike as found by Anquetin et al. earlier, was outside Cryptodira and Testudines.

[4] Similarly, a 2021 study found Protostegidae and Sandownidae to possibly be closely related to or even members of Thalassochelydia, although the exact affinities of the clade are still an open question.

The morphology of the limb girdles resembles those of modern sea turtles, suggesting that they swam in a similar manner.