Enhancing the lightweight potential of mobile steel structures by applying high-strength steels and reducing sheet thicknesses leads to a significant increase of energy effectiveness and a reduction of noxious emissions during operation. However, due to this increase of yield and tensile strength, fracture toughness decreases and notch sensitivity rises. Hence, the local weld geometry becomes more important, especially in case of ultra high-strength steels. This paper deals with the detection and assessment of common geometric weld defects, such as undercuts, and their effect on the fatigue strength of ultra high-strength steel joints. For this purpose, butt joint specimens are welded incorporating ultra high-strength steel as base material. All specimens are judged by visual testing and the detected weld defects undergo an additional surface topography measurement prior to fatigue testing. First, an image processing based Matlab©-Routine is built up to evaluate the local geometrical properties of the weld toe including undercuts. Second, a numerical model of the actual weld geometry is generated. This is utilized to perform numerical analyses in order to compute the actual stress concentration factors as well as fatigue parameters in terms of notch stresses. The experimental work covers fatigue tests of undercut-imperfected and defect-free specimens in order to contribute to the effect of such defects on fatigue life. Finally, an enhanced fatigue assessment of welds with undercuts and high-quality joints is performed based on numerical investigations and validated by experimental results