Antimicrobial resistance (AMR) poses an escalating global health threat, with projections of up to 10 million annual deaths by 2050 unless alternative therapeutic strategies are developed. Green nanotechnology offers a sustainable approach to combating multidrug-resistant (MDR) pathogens by harnessing plant-derived phytochemicals as biocompatible reducing and capping agents. This study aims to synthesize, characterize, and evaluate the antibacterial efficacy of biogenic aluminum–potassium hybrid nanoparticles (P@Al-KNPs) fabricated using aqueous leaf extract of Polyalthia longifolia against MDR clinical isolates of Staphylococcus aureus, Streptococcus sp., and Salmonella sp. P@Al-KNPs were synthesized by reacting aqueous P. longifolia leaf extract (30 mL) with a mixed AlCl₃/KCl precursor solution (70 mL) at 60 °C–70 °C (pH 6.2). Physicochemical characterization employed UV–visible (Vis) spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis. Antibacterial activity was assessed using the Kirby–Bauer disc diffusion method at concentrations of 50, 75, 100, and 150 µg/mL. Data were analyzed using one-way ANOVA with Duncan’s multiple range test at α = 0.05 using SPSS v.19. Successful nanoparticle formation was confirmed using a colorimetric transition and UV–Vis absorption at 408 nm (absorbance: 0.970). ATR-FTIR identified phenolic, carbonyl, and hydroxyl groups as principal mediators of biosynthesis. SEM revealed a predominantly spherical morphology (70–103 nm), and EDX confirmed Al (49.11%) and K (24.99%) as the primary constituents. Antibacterial testing demonstrated significant dose-dependent inhibition zones of 16.4 mm (S. aureus), 16.0 mm (Streptococcus sp.), and 14.3 mm (Salmonella sp.) at 150 µg/mL. The one-way ANOVA reported significant differences among concentrations (p < 0.05). In conclusion, P@Al-KNPs constitute a novel, phyto-mediated dual-metal nanoplatform with significant antibacterial activity against MDR pathogens. Their facile green synthesis and superior activity at subminimum inhibitory concentrations position them as promising candidates for biomedical antimicrobial applications.