An X-ray detector according to an embodiment is an X-ray detector configured by arranging a plurality of detector modules, and includes a first detector module, a second detector module, a voltage application circuit, a first transmission circuit, and a first reception circuit. The second detector module is adjacent to the first detector module among the detector modules. The voltage application circuit is disposed in each of the detector modules, and applies a voltage to a plurality of detection elements included in each of the detector modules. The first transmission circuit is disposed in the first detector module, and transmits a signal based on the voltage from the voltage application circuit of the first detector module. The first reception circuit is disposed in the second detector module, and receives a signal transmitted from the first transmission circuit.