The increasing prevalence of multidrug-resistant (MDR) Pseudomonas aeruginosa highlights the need for alternative antimicrobial agents. This study investigated the antibacterial activity of ethanolic extract derived from red onion peel, an agricultural waste product, against clinical MDR P. aeruginosa isolates. Twelve clinical isolates were identified using standard microbiological and biochemical methods. Antibacterial activity was evaluated using agar well diffusion, broth microdilution, and OD₆₀₀-based spectrophotometric assays. The extract was tested at concentrations ranging from 100 to 0.195 mg/mL. The results demonstrated a strong concentration-dependent antibacterial effect. At 100 and 50 mg/mL, bacterial growth inhibition exceeded 90% across all isolates. At 25 and 12.5 mg/mL, a marked inter-strain variability in susceptibility was observed. Concentrations below 6.25 mg/mL showed negligible activity, comparable to the negative control (p > 0.05). The minimum inhibitory concentration (MIC) was defined using a modified IC90 (≥90% growth inhibition) approach due to the optical interference of crude plant extracts, with effective inhibitory activity observed mainly between 12.5 and 50 mg/mL. Overall, red onion peel ethanolic extract exhibited significant dose-dependent antibacterial activity against MDR P. aeruginosa, supporting its potential as a valorized bioactive waste-derived antimicrobial agent.