Plagiaulax is a genus of mammal from the Lower Cretaceous of Europe.

It was a member of the also extinct order Multituberculata, and shared the world with dinosaurs.

The genus was named by Hugh Falconer in 1857, and was the first described multituberculate species.

Fossil remains of the species Plagiaulax becklesii are known from the Lulworth Formation of Durlston Bay in Dorset, England.

They include at least one partial lower jaw with teeth, though there may well be further specimens.