According to one embodiment, a radiation detector includes a first conductive layer, a second conductive layer, and a first layer. The first layer is provided between the first conductive layer and the second conductive layer. The first layer includes a first region and a second region. The first region includes a metal complex including a first metallic element. The second region includes an organic semiconductor material. The first metallic element includes at least one selected from the group consisting of Ir, Pt, Pb, and Cu.