In this study, post-welding aging treatments were applied to a novel Ni-based superalloy GH4065A inertia friction welding (IFW) joint to improve its high-temperature properties. The effect of aging treatment on the microstructure and creep resistance of the IFW joint was systematically investigated. The results indicated that the originalγ′precipitates in the weld zone almost completely dissolved during the welding process, and fine tertiaryγ′precipitated during the subsequent cooling process. Aging treatment did not significantly change the characteristics of grain structures and primaryγ′in the IFW joint. After aging, the size of tertiaryγ′in the weld zone and secondaryγ′in the base material increased, but their morphology and volume fraction did not change evidently. After 760 °C, 5 h aging treatment, the tertiaryγ′in the weld zone of the joint grew from 12.4 nm to 17.6 nm. Correspondingly, the creep rupture time of the joint at 650 °C and 950 MPa increased from 7.51 h to 147.28 h, which is about 19.61 times higher than that of the as-welded joint. The creep rupture was more likely to occur in the base material instead of the weld zone for the IFW joint. This revealed that the creep resistance of the weld zone was significantly improved after aging due to the growth oftertiaryγ′. However, increasing the aging temperature or extending the aging time promoted the growth of secondaryγ′in the base material, and meanwhile, M23C6carbides tended to continuously precipitate at the grain boundaries of the base material. It might decrease the creep resistance of the base material.