A radiation detector according to an embodiment includes a scintillator array, a sensor array, electronic circuitry, a switch, and control circuitry. The scintillator array includes a plurality of scintillator pixels each configured to convert radiation into light. The sensor array includes a plurality of detection elements each configured to detect the light. The electronic circuitry is configured to output digital data on the basis of signals output from the detection elements. The switch is provided between the sensor array and the electronic circuitry. The control circuitry is configured to control the switch on the basis of a positional relation between the sensor array and the scintillator array.