An apparatus includes a body, a shaft, an ultrasonic element, and an annular clamp pad. The shaft extends distally from the body. The ultrasonic element is located at the distal end of the shaft. The ultrasonic element includes a distally presented annular face. The clamp pad is movable toward the distally presented annular face of the ultrasonic element. When tissue is compressed between the clamp pad and the distally presented annular face of the ultrasonic element, the ultrasonic element may be activated with ultrasonic energy. The resulting ultrasonic vibrations may sever and seal the tissue captured between the distally presented annular face and the clamp pad, resulting in an anastomosis. The apparatus may thus be used to join two hollow tubular tissue structures, such as portions of a gastrointestinal tract.