A thread trimming device for a sewing machine in which a movable knife coupled to a thread trimmer lever by way of a link mechanism is turned to cut sewing threads in cooperation with a fixed knife, the first projection of the thread trimmer lever engaging in the cam groove of a thread trimmer cam, comprises a thread trimmer positive motion bracket having a second projection capable of engaging in the positive motion cam groove of a thread trimmer positive motion cam so as to keep the engagement of the first projection with the cam groove after the second projection thereof engages the positive motion cam groove until it is disengaged therefrom. Since the time of completing the thread trimming can be set by the positive motion cam groove of the thread trimmer positive motion cam, the timing of the thread trimming can be adjusted with certainty. Furthermore, the operating time of the driving means can be remarkably reduced.