Polycotylidae is a family of plesiosaurs from the Cretaceous. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. Several species survived into the final stage of the Cretaceous, the Maastrichtian around 66 million years ago. The possible latest surviving member Rarosaurus from the late Maastrichtian is more likely a crocodylomorph.

With their short necks and large elongated heads, they resemble the pliosaurs, but phylogenetic studies indicate that they are plesiosauroids, being most closely related to Leptocleididae and more distantly to Elasmosauridae. They have been found worldwide, with specimens reported from New Zealand, Australia, Japan, Morocco, the US, Canada, Eastern Europe, and South America.

Cladogram after Albright, Gillette and Titus (2007).

Cladogram after Ketchum and Benson (2010).

Below is a cladogram of polycotylid relationships from Ketchum & Benson, 2011.