No, Leesa mattresses don’t require a box spring. The manufacturer doesn’t even recommend using one.
Instead, it’s best to place your mattress on a hard, stable surface with minimal to no flex.

This includes most platform, slat, and box spring foundations. For slat foundations, check to make sure the slats are no more than 3-4″ apart.
Another option is to use a bunkie board along with your existing bed frame or foundation. Alternatively, you can purchase an adjustable base, mattress foundation, or platform bed directly from Leesa.