ABSTRACT Shewanella spp. constitute a reservoir of antibiotic resistance determinants. In a bile sample, we identified three extended-spectrum-β-lactamase (ESBL)-producing bacteria ( Escherichia coli , Klebsiella pneumoniae , and Shewanella sp. strain JAB-1) isolated from a child suffering from cholangitis. Our objectives were to characterize the genome and the resistome of the first ESBL-producing isolate of the genus Shewanella and determine whether plasmidic exchange occurred between the three bacterial species. Bacterial isolates were characterized using matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS), standard biochemical tools, and antimicrobial susceptibility testing. Shewanella sp. JAB-1 and ESBL gene-encoding plasmids were characterized using PacBio and Illumina whole-genome sequencing, respectively. The Shewanella sp. JAB-1 chromosome-encoded OXA-48 variant was cloned and functionally characterized. Whole-genome sequencing (WGS) of the Shewanella sp. clinical isolate JAB-1 revealed the presence of a 193-kb plasmid belonging to the IncA/C incompatibility group and harboring two ESBL genes, bla CTX-M-15 and bla SHV-2a . bla CTX-M-15 gene-carrying plasmids belonging to the IncY and IncR incompatibility groups were also found in the E. coli and K. pneumoniae isolates from the same patient, respectively. A comparison of the bla CTX-M-15 genetic environment indicated the independent origin of these plasmids and dismissed in vivo transfers. Furthermore, characterization of the resistome of Shewanella sp. JAB-1 revealed the presence of a chromosome-carried bla OXA-535 gene, likely the progenitor of the plasmid-carried bla OXA-436 gene, a novel bla OXA-48 -like gene. The expression of bla OXA-535 in E. coli showed the carbapenem-hydrolyzing activity of OXA-535. The production of OXA-535 in Shewanella sp. JAB-1 could be evidenced using molecular and immunoenzymatic tests, but not with biochemical tests that monitor carbapenem hydrolysis. In this study, we have identified a CTX-M-15-producing Shewanella species that was responsible for a hepatobiliary infection and that is likely the progenitor of OXA-436, a novel plasmid-encoded OXA-48-like class D carbapenemase.