A simple colorimetric assay was developed for the qualitative and semi-quantitative detection of hazardous aluminum(III), Al(III), ions in aqueous solutions, using gallic acid-functionalized silver nanoparticle (GA-AgNPs) probe. The binding of Al(III) ions to GA-AgNPs altered the nanoparticles’ size, morphology, and dispersity, thereby changing the localized surface plasmon resonance and the solution’s color. This effect was thoroughly characterized by systematically studying the influence of GA concentration, pH, and incubation time. The proposed method was validated for its limits of detection and quantitation, and its resilience to potential interferences. This method offers a simple, rapid, and sensitive on-site assay for quantifying low concentrations of Al(III) ions, with results that are both visually and spectroscopically detectable.