To provide a workpiece cutting device for a sewing machine, capable of cutting at any position of a workpiece regardless of whether or not the workpiece is sewn.

The workpiece cutting device 20 for a sewing machine comprises: a fixed frame 41 to be attached to a sewing machine main body 21b; and a movable frame 42 attached to the fixed frame 41 in a movable manner. The movable frame 42 is rotatably provided with a rotary blade 34, and the fixed frame 41 is provided with driving means 37 for driving the rotary blade 34 via a power transmission part 36, and operation means 43 for operating the movable frame 42 so as to move the rotary blade 34 from a non-cutting position P1 of the workpiece to a cutting position P2 of the workpiece. A needle plate 32 provided in a side of a bed part 22 for fixing the sewing machine main body 21b is provided with a fixed blade 38 for cutting the workpiece in cooperation with the rotary blade 34.