The local undercut defects at the weld toe provide a potential initiation site for fatigue cracks and significantly impact the structure's fatigue strength. The influence of continuous undercut depth on fatigue performance is widely studied, but the research on the influence of local undercut geometry is limited. In this research, the influence of the 3D undercut geometry on the fatigue strength of welded joints is investigated using the elasto- plastic finite element analysis, with explicit consideration of geometry and plasticity effect. A simplified parametric model is created to represent the realistic undercut geometry based on high-resolution measurement data. The combination of the geometric parameters for the realistic undercut models was filtered from a full- factorial design of six undercut geometry parameters. The influence of the individual geometric parameters of the local undercut defect such as depth, radius, width, and length on the fatigue performance of weld is systematically investigated. Based on the results, a three-dimensional undercut geometry index is proposed in the current study and found suitable for estimating fatigue crack initiation life. Furthermore, the research provides insights into fatigue strength-based quality criteria for welds with local undercut defects.