The welded Type 304 stainless steel specimens were neutron-irradiated up to 2 × 1025 n/m2 (E > 1MeV) at 561K in the Japan Material Testing Reactor (JMTR). The residual stresses in the test specimens were measured by neutron diffraction, and the radiation-induced stress relaxation was evaluated from the change of stress distributions along the direction transverse to the weld bead. The magnitudes of σX (transverse to the weld) and σY (longitudinal to the weld) residual stresses decreased steadily with increasing neutron fluence. The irradiation dose dependence and the mechanism for the relaxation were examined using an equation derived from the irradiation creep equation. There was a steep stress relaxation, which might correspond to transient creep, at the early stage of neutron irradiation near the weld, and the different radiation-induced relaxation behaviors could be observed depending on the distance from the weld. The radiation-induced stress relaxation for welding-induced residual stress, which was generated under a three-dimensional restriction, indicated the tendency to be slower compared to the fundamental results for mechanically applied stress.