According to one embodiment, a radiation detector includes a scintillator layer, a first conductive layer, a second conductive layer, and an organic layer. The second conductive layer is provided between the scintillator layer and the first conductive layer. The organic layer is provided between the first conductive layer and the second conductive layer. The organic layer includes an organic semiconductor region having a first thickness. The first thickness is 400 nanometers or more.