The aim of this article is twofold: first, to show that, in Plato's Hippias Major, Hippias is the mouthpiece of a materialist ontology; second, to discuss the critique of this ontology. My argument is based on an interpretation of Hippias Major 300b4–301e3. I begin by revealing the shortcomings of P. Woodruff's and I. Ludlam's interpretations. Next, I define the concept of materialism as it was understood in ancient Greece (Democritus) in order to outline the specificity of Hippias' materialism. Finally, I argue that the opposition between the two characters of the Hippias Major represents in fact an ontological opposition between two conceptions of what a unity is, i.e., Hippias' elementary corporal unities and Socrates' “formal unity.”