A thread cutter for a sewing machine includes a first thread seizing assembly reciprocally movable and including two unit thread seizing members having distal ends formed with first thread seizing portions respectively, a cutting blade located nearer to the needle hole side than a movement locus of the first assembly, and a second thread seizing assembly seizing the needle and bobbin threads both seized by the first assembly during backward movement of the first assembly, cutting the threads in cooperation with the cutting blade. When the threads seized by the first assembly are further seized by the second assembly, the first assembly is moved so that the distal ends of the unit thread seizing members are spaced from each other by a predetermined distance in a direction intersecting a movement direction of the first assembly.