An ultrasound diagnostic apparatus includes processing circuitry. The processing circuitry generates a first image based on an echo signal obtained by transmission and reception of ultrasound waves. The processing circuitry acquires a second image that is an image generated by a medical image diagnostic apparatus. The processing circuitry performs a registration of the first image and the second image, by discretely setting a plurality of relative positions of the first image and the second image within a specified range, calculating the similarity between the first image and the second image corresponding to the plurality of relative positions respectively, updating the plurality of the relative positions based on the calculation result of the similarity, and recalculating the similarity corresponding to the plurality of the updated relative positions respectively. The processing circuitry causes a display to display the images obtained after the registration.
