An X-ray computed tomography apparatus according to embodiments includes image processing circuitry and decomposition circuitry. The image processing circuitry is configured to perform an image processing on each of a plurality of pieces of monochromatic X-ray image data of different energies, the plurality of pieces of monochromatic X-ray image data being generated from projection data. The decomposition circuitry is configured to decompose, for each of a plurality of basis materials specified in advance, the plurality of pieces of monochromatic X-ray image data after the image processing, to generate basis material image data of each of the plurality of basis materials.
