Acinetobacter baumannii is frequently associated with nosocomial infections, and its increasing resistance to carbapenems is a concern. The blaOXA-23 gene encoding carbapenem-hydrolyzing class D β-lactamase (CHDL) has been detected in many clinical A. baumannii isolates worldwide, with all CHDL-positive isolates previously found in hospital settings. In this study, we describe the first A. baumannii strain (B9) isolated from the environment (water of the Seine River in downtown Paris) that produces an acquired CHDL. Isolate B9 was identified via the API 32GN system and 16S rRNA sequencing. MIC determination showed it was resistant to all β-lactams including carbapenems, quinolones, fluoroquinolones, chloramphenicol, tetracycline, and tigecycline, but susceptible to aminoglycosides. PCR and sequencing confirmed the presence of the blaOXA-23 gene. Failed attempts to transfer the determinant via mating-out assay and electroporation, along with negative plasmid extraction results, suggested chromosomal location of blaOXA-23, which was verified by I-CeuI digestion, pulsed-field gel electrophoresis, and hybridization. PCR also revealed a structure where ISAba1 was upstream of blaOXA-23. This is the first isolation of an environmental A. baumannii strain producing an acquired CHDL. The progenitor of blaOXA-23 is Acinetobacter radioresistens (an environmental species with poorly expressed blaOXA-23), leading to the hypothesis that genetic exchanges between A. baumannii and A. radioresistens in aquatic environments may have enabled acquisition and expression of blaOXA-23 in A. baumannii. PFGE showed B9 was clonally related to a clinical A. baumannii isolate from New Caledonia. Our study indicates the spread of CHDL-producing A. baumannii is not restricted to hospital settings and emphasizes the importance of surveying environmental strains, which may act as sources or reservoirs of clinically relevant resistance genes.