Effects of He irradiation on polycrystalline yttria-stabilized zirconia (YSZ) are studied with the focus on irradiation-induced damage buildup, He behavior, and volume swelling. The evolution of irradiation-induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three-step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single-crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single-crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.