Background: Currently, an issue of increasing concern to public health authorities is the spread of multiresistant extended-spectrum β-lactamase (ESBL) - and carbapenemase-producing Enterobacteriaceae in the environment, mainly through aquatic ecosystems. Osona, a county in Catalonia, in northeastern Spain, has a high density of pig, goat and sheep farms as well as high employment in this sector and is therefore at high risk of this kind of biocontamination. Methods: Over the 2020–2023 study period the aims of our study were (a) to analyse the prevalence of ESBL- and carbapenemase-producing Enterobacteriaceae in water from several rivers flowing through Osona as well as all the wastewater treatment plants (WWTPs) located in it; (b) to genetically characterize all ESBL- and carbapenemase-producing Enterobacteriaceae strains detected; and (c) to compare ESBL-producing E. coli environmental and human community strains to check for a common origin. Results: Multiresistant Enterobacteriaceae were detected in 15 out of 31 (48.4%) WWTPs, mainly ESBL-producing E. coli and an OXA-48-producing K. pneumoniae strain. Regarding river water, 4 strains of ESBL-producing E. coli and a strain of VIM-producing E. cloacae complex were isolated from 29 samples (17.2%). Finally, rectal carriage of ESBL-producing E. coli was detected in 66.7% of the Enterobacteriaceae-positive swab samples. Conclusions: These findings show a significant presence of ESBL- and carbapenemase-producing Enterobacteriaceae in water from rivers and WWTPs in Osona. The persistence of multidrug-resistant Enterobacteriaceae in WWTP effluents points to insufficient clearance during treatment processes. Several environmental and human strains of the multiresistant microorganisms identified in this study have a common ancestor, which suggests a similar origin.