An X-ray CT system according to an embodiment includes processing circuitry configured to: execute first scanning of acquiring a first subject data set corresponding to first X-ray energy by irradiating a first region of a subject with X-rays; execute, after the first scanning, second scanning of acquiring a second subject data set corresponding to second X-ray energy and a third subject data set corresponding to third X-ray energy different from the second X-ray energy by irradiating a second region included in the first region with X-rays; and perform material decomposition among a plurality of reference materials based on: a fourth subject data set obtained based on the first subject data set and one of the second subject data set and the third subject data set; and the other of the second subject data set and the third subject data set.