Au-decorated ZnIn₂S₄ plasmonic micro-architectures were synthesized through the hydrothermal method, followed by a citrate reduction process that eventually facilitates the formation of hierarchical microsphere porous morphologies for effective catalysis. An optimum amount of 2.5 wt% Au/ZnIn₂S₄ exhibits a high degree of photochemical conversion in both toxic organic dyes and antibiotics, compared to that exhibited by pristine ZnIn₂S₄. Photoluminescence and photocurrent density confirms the charge transfer mechanism, and the electron spin trapping confirms the formation of radical species. The improved photoelectrocatalytic performance is demonstrated by the rapid charge separation, transmission, and availability of more reactive species from localized Au/ZnIn₂S₄ surface plasmon resonance at an optimum bias voltage of 0.5 V. The designed sensor exhibits a reasonably high sensing ability with a low limit of detection (0.8 μM) within the linear concentration range from 0.5 to 250 μM. This research highlights the improved strategy for developing noble metal decorated ternary metal sulfides and synchronizes photoelectrochemical properties to extend its application as sensors and photoelectrocatalysts.