<p>Magnesium (Mg) alloys, particularly those alloyed with strontium (Sr) and zirconium (Zr), have attracted considerable attention for lightweight structural applications due to their excellent strength-to-weight ratio. However, improvements in mechanical performance are still required for broader industrial utilization. This study investigates the influence of yttrium (Y) addition (0–8 wt.%) on the mechanical properties and microstructure of a cast Mg–2Sr–0.5Zr alloy. Microstructural characterization using X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM) revealed that zirconium acts as the primary grain refiner, while strontium contributes to the formation of thermally stable intermetallic phases such as Mg₁₇Sr₂ along grain boundaries. Yttrium addition leads to the formation of Mg₂₄Y₅ intermetallic, which becomes pronounced beyond 4 wt.% Y. While Y addition significantly enhances strength, it simultaneously reduces ductility. Tensile and hardness tests demonstrated significant enhancements in yield strength (YS) and ultimate tensile strength (UTS) up to 4 wt.% Y, accompanied by a reduction in ductility, attributed to the combined effects of dispersion hardening and solid solution strengthening. The UTS peaks at 4 wt.% Y, while YS continues to increase slightly up to 8 wt.% Y. The optimal Y content was found to be 4 wt.%, yielding a UTS of 136.6 MPa, a YS of 115.4 MPa, and an average grain size of 35.16 µm, while further Y addition (8 wt.%) led to intermetallic coarsening and a drastic loss in ductility. This trade-off between strength and ductility is governed by the percolation of brittle Mg₂₄Y₅ intermetallics, which become pronounced beyond 4 wt.% Y. The results indicate that controlled yttrium addition effectively strengthens the Mg–2Sr–0.5Zr alloy, though with a sacrifice in ductility, making it a promising material for high performance lightweight structural components where formability is not the primary constraint.</p> Graphical Abstract <p></p>