Several Upper Cretaceous plesiosaur specimens recovered from southernmost Chile are described here. These were collected from upper levels of the Dorotea Formation exposed on three different localities (Sierra Baguales, Cerro Castillo, and Dumestre). The new material includes the first record of Aristonectes (Plesiosauria, Elasmosauridae), previously recorded from Argentina, central Chile, and Antarctica. Additional specimens include associated postcranial skeletons as well as isolated elements. Among these, we recognize the presence of aristonectines in the three studied localities, while non-aristonectine elasmosaurids were only collected from Cerro Castillo. The specimen from Dumestre is remarkable by being a small-sized adult, indeterminate aristonectine, and could be related to known representatives from Antarctica. These new finds prove the abundance of aristonectines as well as intermediate elasmosaurids along the Magallanes Basin during the uppermost Cretaceous, while extreme long-necked elasmosaurids as well as polycotylids seems to be completely absent during this time span. This key record from southernmost Chile and its strategic placement in the middle part of the Weddellian Province gives the chance for complementing the paleobiogeography of Upper Cretaceous plesiosaurs from the Southern Hemisphere. As a first result, a faunal turnover is observed during the early Maastrichtian, when extreme (very-long necked) elasmosaurids and polycotylids disappeared from the austral record. Since the early Maastrichtian and towards the late Maastrichtian, aristonectines became differentially abundant along the southeastern Pacific and Antarctica, but moderately represented in the southwestern Atlantic. On contrary, intermediate elasmosaurids were scarce in the Antarctic-Pacific realm, but abundant in the Atlantic. The updated record of austral plesiosaurs suggest a first stage of interchange from the Northern into the Southern Hemisphere, and through the Atlantic seaway, at least since the Coniacian to the late Campanian-early Maastrichtian. During the early Maastrichtian, aristonectines were relatively frequent in the New Zealand-Antarctica archipelago, becoming abundant along southern South America during the late Maastrichtian.