In philosophy of mathematics, a **foundation ontology** is an ontology in the formal philosophical sense that is deemed to play a role in the foundations of mathematics. Most notably, the role played by Plato's ontology in some theories of realism in mathematics. Hilary Putnam made the distinction in 1975, arguing that one could believe in a realist philosophy of mathematical foundations without also accepting Plato's ontology or his sacred geometry, thus the labels "Platonist" and "realist" were not to be held equivalent.

This is discussed further in the article on foundations of mathematics.