Abstract: A guard constructed of plastic-clad wire has a horizontal portion captured beneath the corner of a box spring, and vertical portions extending upwardly adjacent the sides of the box spring and a mattress lying thereon. The vertical portions prevent the mattress from sliding over the edges of the box spring at the corner; use of four such guards, one at each corner, immobilizes the mattress and keeps it properly aligned with the box spring. A brace portion is affixed over the box spring to clamp the guard to the box spring and to stiffen the vertical portions of the guard. A second horizontal portion is formed about the corner of the mattress between the ends of the vertical portions to further stiffen the assembly and to assist in retaining the mattress in position.