Electron backscatter diffraction (EBSD) measurements were performed for welded 308 stainless steel corroded under proton irradiation at 473 K. After chemical cleansing and subsequent mechanical polishing to remove oxide layers on the specimen surface for EBSD measurements, selective dissolution of delta ferrite (δ) phase in the dendritic microstructure was detected only for the in-beam corroded specimen. The susceptibility to δ phase dissolution is dependent on the deviation angle of its orientation from the (101) plane on the specimen surface when δ phases are in a grain of austenitic gamma (γ) phase as well as on the γ-γ phase grain boundaries except for the coincidence grain boundaries. Selective dissolution of δ phase would be explained by not only radiolytic products such as radical oxygen acting as electric charge carriers but also the radiation-induced point defects and dislocation structures playing an important role in sustaining anodic reactions at δ phases.