Extended-spectrum-β-lactamase (ESBL) and carbapenemase-producing Enterobacterales and Acinetobacter spp. are important nosocomial pathogens that are frequently isolated from patients and food matrices. Nevertheless, comprehensive data on the prevalence, spatiotemporal variations, and characterization of β-lactam-resistant bacteria in poultry meat products are limited. This study provides the first comprehensive assessment in Greece of the prevalence, characteristics, and proteomic relationships of β-lactam-resistant strains in poultry meat at the abattoir level. Strains were selectively isolated using β-lactams and identified via MALDI-TOF MS. Antimicrobial susceptibility and the presence of common β-lactamase genes were assessed, and protein profiles were analyzed to determine strain relationships, whereas E. coli isolates were further classified into phylogenetic groups. The overall prevalence was 40.8% for E. coli, 3.3% for K. pneumoniae, and 46.7% for Acinetobacter spp., with notable seasonal and regional fluctuations especially in Acinetobacter spp. Most strains (97.9% of E. coli, 100.0% of K. pneumoniae and 88.1% of Acinetobacter spp.) were classified as multidrug or extensively drug-resistant. All E. coli and K. pneumoniae strains were phenotypically confirmed as ESBL/AmpC producers, whereas one K. pneumoniae strain showed additional resistance to ertapenem. The majority of E. coli strains (91.49%) and all K. pneumoniae strains carried β-lactamase genes, predominantly bla