A radiation detector according to an embodiment includes: a cathode electrode, a plurality of anode electrodes, a crystal, an anti-scatter grid, and a conductive part. The cathode electrode is provided on the radiation incident side. The plurality of anode electrodes are arranged so as to oppose the cathode electrode. The crystal is provided between the cathode electrode and the plurality of anode electrodes and configured to convert incident radiation into electrons. The conductive part is provided between the plurality of anode electrodes and has a conductivity higher than that of the crystal. The anti-scatter grid is provided on the radiation incident side of the cathode electrode so as to oppose the conductive part via the crystal, while being arranged in a first direction.