According to one embodiment, a radiation detector includes a first layer, a first light-emitting part, a detecting part, a detection circuit, and a first drive circuit. The first layer includes a first organic material. The first light-emitting part includes a first organic light-emitting layer. The detecting part is provided between the first layer and the first light-emitting part. The detecting part includes an organic photoelectric conversion layer and is configured to generate an electrical signal corresponding to radiation incident on the first layer. The detection circuit is configured to output a detection signal based on the electrical signal. The first drive circuit is configured to supply a first drive signal to the first light-emitting part based on the detection signal.