An infant mattress fits safely in a crib without any gap between the mattress and the walls or bars of the crib. A ring of memory foam surrounds the supporting foam layers of the mattress and forms a snug fit with the vertical bars of the crib regardless of whether the dimensions of the particular crib or mattress vary from their standard sizes. The memory foam ring intentionally renders the infant mattress somewhat larger than the inner dimensions of the crib. The ring compresses when the mattress is placed in a crib in which opposing side bars are separated by a distance that is smaller than the mattress width. The ring is formed by pouring foam into a rectangular mold around the supporting foam layers. Where the mattress includes pocket coils on the supporting foam layers, the ring of memory foam conforms to the outer row of pocket coils.