Gallium oxide (Ga2O3) is attractive for power devices owing to its wide bandgap of 4.5 eV and the availability of economical device-quality native substrates. Research on Ga2O3 Schottky barrier diodes and field-effect transistors (FETs) has seen rapid recent progress [1]. An unexplored area of immense interest is the radiation tolerance of these devices, whose high-voltage and high-temperature capabilities are expected to find applications in extreme radiation environments such as space and nuclear facilities that impose stringent reliability requirements to ensure stable operations. This paper reports the first investigation into the effects of ionizing radiation on Ga2O3 metal-oxide-semiconductor FETs (MOSFETs). A gamma-ray (y-ray) tolerance as high as 230 kGy(SiO2) was demonstrated for the bulk Ga2O3 channel by virtue of the MOSFETs' stable on-current, on-resistance (Ron), and threshold voltage (Vt) against irradiation. Hysteresis in the transfer characteristics remained negligible after exposure to the highest dose. Radiation-induced degradations in the gate insulation and surface passivation, which could be attributed to dielectric damage and interface trap generation, were found to limit the overall radiation resistance of these devices.