The development of efficient and selective nanocatalysts for biomass valorization is critical for advancing sustainable chemistry. Here, we report the colloidal synthesis of spherical gold nanoparticles (AuNPs) coated with manganese oxide (MnO $$_2$$ ) shells of tunable thicknesses (3.5, 5.0, 7.5, and 10.0 nm), forming Au@MnO $$_2$$ core-shell nanostructures with well-preserved plasmonic properties. Structural and spectroscopic characterization confirmed the successful fabrication and a red-shifted localized surface plasmon resonance (LSPR), indicative of MnO $$_2$$ shell formation. Supported on ZrO $$_2$$ , the Au@MnO $$_2$$ catalysts exhibited enhanced activity and selectivity in the photocatalytic oxidation of 5-hydroxymethylfurfural (HMF) under 365 nm and 450 nm light, outperforming thermal conditions. The MnO $$_2$$ shell thickness and irradiation wavelength significantly influenced product distribution, favoring valuable oxidation intermediates. In addition to improved catalytic performance, the core-shell design provided enhanced stability. These findings highlight the potential of plasmonic core-shell nanostructures for light-driven biomass conversion and broader nanocatalysis applications.