Neutron-irradiated tungsten (W) samples were exposed to helium (He)–seeded deuterium (D) plasmas using a linear plasma device called Tritium Plasma Experiment in order to investigate the synergetic effects of neutron and He irradiations on D retention in W. Exposure to nonseeded D plasma was also performed for neutron-irradiated and nonirradiated W samples for comparison. Deuterium retention in neutron-irradiated W after D plasma exposure was two to three times larger than that in W without neutron irradiation. Nevertheless, He seeding in D plasma resulted in a drastic reduction in D retention. The cross-sectional observation by transmission microscopy showed formation of He bubble layers with a thickness of 10 to 20 nm. There is a possibility that alpha particles in fusion plasma reduce tritium retention in neutron-irradiated plasma-facing components with W layers.