HCM12A is an advanced 12 Cr ferritic-martensitic steel designed for higher temperature operation than could be achieved using earlier generation steels such as HT9. HCM12A is one of the advanced alloys under consideration for application in core components in Generation IV nuclear energy systems, and is of particular interest to the supercritical water reactor, sodium fast reactor, and lead fast reactor designs. The radiation resistance of HCM12A has not previously been studied. This work provides information on the hardening and microstructural changes in HCM12A after irradiation using 2.0 MeV protons at 400°C to 10 dpa and 5 MeV Ni-ions at 500°C to 50 dpa. Following irradiation, changes in hardness were measured using Vickers hardness indentation, changes in microstructure and phase stability were studied using transmission electron microscopy, and changes in microchemistry were measured using scanning Auger microscopy. The hardness increases by roughly 70 % and saturates by roughly 5 dpa. The changes to the microstructure contributing to this hardness increase are primarily due to the formation of precipitate phases, with some contribution from changes in dislocation density. Chromium is enriched at grain boundaries prior to irradiation, likely due to grain boundary carbides, and increases further during the irradiation.