The invention relates to a specially-designed corner piece for mattresses comprising springs (4). The inventive corner piece consists of: a cylindrical tubular body (1, 1') comprising a longitudinal parting line (2), which corresponds to one of the generatrices thereof and which connects the two ends of same; and a plurality of wide recesses (3, 3') which open out towards said ends, thereby forming an isosceles trapezium. The aforementioned recesses, together with the longitudinal opening (2), can be used to provide a tongue-and-groove connection between the corner piece and the springs (4) of the mattress. In this way, the spring assembly is sealed off externally at the corresponding corner and a continuous, smooth surface is produced without any bumps, dents or deformations.