Serratia marcescens is a nosocomial pathogen with carbapenem resistance, which limits the availability of effective treatment options. In this study, we performed molecular characterization of GES-5 carbapenemase-producing S. marcescens isolated from an outbreak in Japan. Comparative genetic analysis revealed that the blaGES-5-encoding plasmid p2020-O-9 is a unique plasmid contributing to carbapenem resistance. Furthermore, this study highlights the necessity of surveillance programs for monitoring novel, along with commonly occurring carbapenemases in clinical settings.