The strain and stress build-up in 20-keV He-implanted UO2 single crystals have been determined by means of X-ray diffraction through reciprocal space mapping, with the use of a model dedicated to the analysis of the strain/stress state of ion-irradiated materials. Results indicate that the undamaged part of the crystals exhibits no strain or stress; on the other hand, the implanted layer undergoes a tensile strain directed along the normal to the surface of the crystals and a compressive in-plane stress. The build-up of both strain and stress with He fluence exhibits a two-step process: (i) a progressive increase up to a maximum level of similar to 1% for the strain and similar to-2.8 GPa for the stress, followed by (ii) a dramatic decrease. The origin of the strain and stress build-up is the formation of both self-interstitial defects and small He-vacancy clusters. The strain, and stress relief is tentatively attributed to the formation of extended defects (such as dislocations) that induce a plastic relaxation.