The macroalga Sargassum siliquosum contains amino-cellulose bioactive compounds that act as stabilizing agents and growth controllers for metal precursor ions in the synthesis of Mg-Y/ZnO nanorods. This study aims to improve the multifunctional biomedical performance of ZnO (antibacterial, antioxidant, anti-inflammatory, and antidiabetic) through bimetal ion doping using Magnesium (Mg2+, 0.71 Å) and Yttrium (Y3+, 1.05 Å) at concentration ratios of 0.01–0.03 M within the ZnO lattice. Thermal analysis (TGA/DTA) shows that Mg-Y doping reduces the thermal stabilization temperature of ZnO from 800°C to 600°C. X-ray diffraction confirms the formation of a hexagonal wurtzite structure without secondary phases, supported by Rietveld refinement, FT-IR bonding profiles, and UV-Vis (Tauc plot) band gap narrowing to 3.01–3.10 eV. FE-SEM images reveal nanorod morphology of 25–35 µm, while EDX verifies Mg and Y incorporation. The doped samples exhibit strong antibacterial activity, with inhibition zones of ≥ 20 mm against Staphylococcus aureus and Pseudomonas aeruginosa. The 0.03 M Mg-Y/ZnO sample exhibits the highest biomedical performance, with significant antioxidant (IC₅₀ = 71.6 mg/L), anti-inflammatory (368 mg/L), and antidiabetic (420 mg/L) activities. These results indicate that Mg-Y bimetal doping enhances ZnO functionality and offers promising potential as a multifunctional biomedical material. © 2025