A frame (1) for sewing upper pillows on covering shells for mattresses, which comprises four perimetric walls (2). Each wall (2) is perpendicular to the two contiguous walls (2) and comprises at least one laminar portion (3) that constitutes a resting surface: the laminar portion (3) is co-planar with a lower edge of the wall (2); at least one of the walls (2) comprises at least one locking element (4) for at least one component, selected from between an upper pillow and a covering shell, arranged on a respective laminar portion (3); at least one of the walls (2) comprises at least two bands (5, 6) and (7, 8) that are mutually aligned and can slide with respect to each other in order to vary the overall length of the wall (2); a linear actuator (9, 10) is interposed between the at least two bands (5, 6) and (7, 8) of the wall (2), for the controlled mutual sliding of the bands; the actuator (9, 10) being adapted to be controlled by a respective control and management unit of a sewing machine (100) that is functionally associatable with the frame (1).