This work focuses on the prediction of the multiaxial fatigue limit at constant amplitude for metallic materials weakened by stress concentrators. Developing predictive models for multiaxial fatigue limits, which enable the analysis of the effects of defects and notches, is especially relevant in the context of the rapid advancement of Additive Manufacturing (AM) technologies. This is due to two main reasons: (i) current AM processes are inherently prone to defects, and (ii) AM technologies allow manufacturing components with complex geometries, such as lattice structures, characterized by the presence of notches and multiaxial stress states. More specifically, the study addresses the engineering estimation of the multiaxial fatigue limit in the presence of defects, cracks, and sharp U or V notches and proposes a unified design criterion based on the Averaged Strain Energy Density (SED) approach, which extends the Atzori-Lazzarin-Meneghetti (ALM) diagram to multiaxial loadings. The SED framework offers a method to define both an equivalent defect-free material fatigue limit, denoted as Δσ0eq, and an equivalent fatigue threshold, for any local stress field. By combining these two parameters using an El Haddad Smith Topper-type equation, defect sensitivity under local multiaxial stresses can be described. An extensive validation against experimental multiaxial fatigue limits is included, demonstrating the sound correlation between theoretical estimations and experimental results. This validation highlights the robustness and reliability of the proposed method in view of real-world applications, where understanding the impact of stress raisers, such as defects and notches, is critical to ensure the structural integrity of mechanical components.