A radiation detector according to an embodiment includes a plurality of detector modules, a first and second radiation shield, and first supporter. The first radiation shield is provided in a first detector module and is arranged on a side opposite to a surface of a first detector pack of a first detector module on which radiation is incident. The second radiation shield is arranged to intersect with a path of radiation that passes through between a first detector pack and a second detector pack of a second detector module that is arranged adjacently to the first detector module. The first supporter supports the first radiation shield ouch that at least a part of the first radiation shield overlaps the second radiation shield on the path of radiation.