The hydrothermal corrosion behavior (320 °C, 20 MPa, 168 h) of high-purity chemical-vapor-deposited (CVD) SiC pre-irradiated with 5.1-MeV Si ions at 400 and 800 °C and 0.1-2.6 dpa was studied in order to clarify the effects of irradiation damage on SiC corrosion. Regardless of the pre-irradiation conditions, selective corrosion was observed at the grain boundaries and stacking faults even at the unirradiated regions. In contrast to the complete loss of the irradiated regions observed in the specimens irradiated at 400 °C during the autoclave test, a number of large grains survived in the case of the specimens irradiated at 800 °C. The corrosion rates at the irradiated regions increased with increasing irradiation fluence, with a significant dependence in the lower dpa regime similar to that observed in the point-defect swelling. SiO2 formation was not detected in any case. Cross-sectional scanning transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses of the surfaces of the surviving grains revealed oxygen diffusion to a depth of 3.0 nm from the surface. A significant reduction of the oxygen diffusion barrier at the surface was implicated as one of the key mechanisms of the acceleration of the ion-irradiated SiC corrosion rates.