Does this plush foam mattress need a box spring?

No box spring is required. This 9.5-inch all-foam mattress has a dense Bio Comfort foam core that supports itself on any firm, flat surface: a platform bed, a slatted base with gaps under 3 inches, or a bunkie board over an old box spring. A traditional spring box spring flexes and can create soft spots under a foam bed, so a solid flat foundation is the better choice and keeps the 10-year warranty valid.