Reptiles constitute a primarily terrestrial assemblage, but several groups returned to the marine environment after the first appearance of reptiles in the late Palaeozoic era. Successful diversification of the chelonioid sea turtles, particularly during the Cretaceous period, was perhaps one of the most important events in the history of turtles (and marine reptiles). The fossil record of chelonioids before the Late Cretaceous has been poorly documented. Here I report the discovery of an exceptionally well-preserved skeleton of the oldest known chelonioid, from the Early Cretaceous stage (about 110 million years before the present) of eastern Brazil. This specimen represents a new taxon, extending the history of chelonioids by 10 million years, and it sheds new light on the early evolution of the group. The limb of the specimen is a relatively primitive paddle, which still possesses movable digits as in freshwater turtles. However, the skull is specialized in the manner of later chelonioids, with large interorbital foramina that are indicative of huge lachrymal salt glands surrounding the eyes,. This discovery supports the idea that the establishment of the salt-excreting system, and the occupation of a marine habitat, may have preceded the formation of rigid paddles in the history of chelonioids.