Abstract Stress-controlled fatigue tests were performed for He-pre-injected 316 stainless steels under in situ 17MeV proton irradiation at 573K. The pre-injection of He for fatigue specimens was conducted with 20MeV α particles to a level of 100appm at 573K. The hydrogen implantation rate under 17MeV proton irradiation was 7.1×10−4appm/s at a dose rate of 1.1×10−7dpa/s. The number of fatigue cycles to fracture (NF) increased slightly under in situ irradiation for specimens without He-pre-injection, while a substantial decrease in NF was detected under in situ irradiation for the He-pre-injected specimens. Although the fatigue behavior under in situ irradiation appears to be explained by the in situ irradiation effect based on the interaction between radiation-induced defect clusters and moving dislocations for specimens without He-pre-injection, the implanted hydrogen could play a role in reducing the in situ irradiation effect on the fatigue behavior for the He-pre-injected specimens, even at higher temperatures such as 573K.