The fatigue behavior of a 4-mm thick laser-hybrid welded structure was studied using small- and full-scale specimens. The aim of the work was to understand the response and fatigue strength of large thin welded structures. The difference and similarity between small- and full-scale specimens, which is crucial in order to transfer fatigue test results into fatigue design, was carefully studied. The experiments included accurate optical geometry measurements and constant amplitude fatigue testing under axial loading. The fatigue test results were analyzed in terms of structural hot spot stress. The results showed that when initial distortion shape and geometrical nonlinearity are properly considered, the small- and full-scale specimens have equal fatigue strength with small scatter and the same S-N curve slope close to m = 5. In addition, the measured fatigue strength is considerably higher in comparison to IIW structural stress design curve FAT100. This indicates that high fatigue strength can be achieved in thin laser-hybrid welded structures, given that the shape and the magnitude of initial distortion as well as the weld quality are controlled.