It depends on your current bed frame. If you have a platform bed with either a solid surface or slats spaced less than 4" apart and a center support for queen and larger sizes, a foundation is not required.
If you don't have a platform bed frame, we generally recommend purchasing one of our foundations to go along with your new Saatva mattress. But if your box spring or foundation is in very good condition—meaning it's sturdy and has absolutely no sagging—it should work fine. Your mattress warranty remains in place even when using your existing foundation or box spring.