An attachment (4) for sewing machines (1) for trimming the material to be sewn has a cutting tool comprising a blade (5) and a counter blade (6), which can be driven in dependence on the intermittent feed of the material to be sewn, and whose cutting plane (S) is aligned parallel to the feed. To achieve an inexpensive, space-saving and safe cutting of the material to be sewn, the cutting tool has a rotary blade (5) to be driven around an axis of rotation (D) normal to the cutting plane (S), which forms at least one cutting edge (7) extending only over part of the blade periphery and along its course deviates from a circular arc around the axis of rotation (D), and which rotates with a number of revolutions adapted to the feed of the material to be sewn indirectly proportional to the number of cutting edges.