Aeromonas spp. are environmental reservoirs of resistance determinants to different classes of antibiotic molecules. The aim of this study was to evaluate the prevalence of extended-spectrum β-lactamases (ESBLs) among Aeromonas sp. strains recovered from several rivers, lakes, and activated sludges from the southern part of the Swiss Alps. Water samples were collected between 2002 and 2005 from the rivers Ticino and Vedeggio, lakes Cadagno and Lugano, and activated sludges in the Tessin county of Switzerland. Fifty Aeromonas sp. strains were recovered and tested for ESBLs via a synergy test using ticarcillin-clavulanic acid, ceftazidime, cefotaxime, and aztreonam disks; susceptibility testing was done with Etest strips. A single isolate (A72) from an activated sludge in Bioggio showed an ESBL phenotype, resistant to most β-lactams except cephamycins and carbapenems. PCR and sequencing identified the blaPER-1 gene, and gyrB sequencing confirmed the isolate as Aeromonas media. A 70-kb plasmid (pAM) carrying blaPER-1 was conjugatively transferred to Escherichia coli J53, with the transconjugant exhibiting a susceptibility pattern matching A72. PCR mapping showed blaPER-1 was part of the Tn1213 composite transposon, flanked by the aphA6 and strA genes (aminoglycoside resistance) from Tn5393d—similar to the genetic context of a PER-1-producing Alcaligenes faecalis clinical isolate from Italy. This study marks the first identification of an ESBL-producing environmental Aeromonas sp. strain. The presence of blaPER-1 on a conjugative, broad-host-range plasmid in a genetic context akin to a neighboring country’s clinical isolate suggests Aeromonas spp. may act as vehicles for spreading this resistance determinant. These findings also strengthen Aeromonas spp.’s role as reservoirs or vectors of clinically relevant antimicrobial resistance determinants, as supported by recent identification of the plasmid-mediated quinolone resistance qnrS2 gene in Aeromonas punctata and A. media from the Seine River, France.