The high clinical burden of antimicrobial resistance (AMR) and the dwindling antibiotic pipeline demand standardised tools to monitor resistance development to new antibiotics. The Emerging Resistance Index (ERI) is a novel quantitative tool designed to track resistance to newly developed antibiotics. The ERI incorporates various parameters: AMR pooled proportions and trends, availability of AMR and antimicrobial consumption surveillance systems, time lag between antibiotic approval and resistance detection, and the average annual number of outbreaks caused by resistant strains. AMR and antimicrobial consumption data on 13 new antibiotics were extracted by reviewing the literature and the reports from 38 surveillance systems included in the European Clinical Research Alliance on Infectious Diseases-The Epidemiology Network central data repository, encompassing 27 EU countries, four European Free Trade Association countries, and the UK. Our analysis revealed rapid resistance development to all new Gram-negative-targeting antibiotics, with particularly high ERI values for imipenem-relebactam and cefiderocol. Combinations of old β-lactams with novel inhibitors were especially prone to rapid resistance development. The ERI could offer a quantitative and dynamic tool for monitoring resistance trends. Also, the ERI could support more timely surveillance efforts, inform antibiotic policy decisions, and aid in prioritising antibiotic research and development.