The Japanese word for mattress is
マットレス

mattoresu

Japanese Definition

mattress
1. 名詞. マットレス

布団
1. n. (from early-mid 1200s) (Buddhism specifically, Zen) a round cushion made of woven bulrush leaves, used for zazen seated Zen meditation
2. n. (from 1638) q, most common sense in modern usage traditional Japanese-style bedding, more specifically the 敷布団 or 掛け布団, or a set of both, spread directly on a tatami floor
布団を敷く - to lay out the futon → to make the bed
布団をあげる - to raise the futon → to put away the bedding
3. n. Usage: * 布団 may refer to just the mattress, just the comforter, or the entire bed consisting of both.
4. n. Usage: * Note that the Japanese term has no "furniture" sense, unlike the borrowed English term futon, which can refer to a specific kind of sofa-bed.