Abstract: An underbed thread trimmer mechanism for severing the needle and bobbin threads upon completion of a sewing operation. Operation of the thread trimmer may be selectively initiated by operator command. A plurality of cams and members cooperate to transfer power from a loop taker shaft to a thread handling disc-like member situated in close proximity to the sewing machine loop taker. A thread cutting knife is disposed in covering relation to the thread handling member and cooperates with the thread handling member to trap the needle and bobbin threads and direct them to a knife surface of the thread cutting knife where they are severed. The mechanism that drives the thread trimmer resets itself at the completion of the thread cutting cycle.