Proteus mirabilis is a Gram-negative bacterium found in the environment and also forms part of the commensal flora in the gastrointestinal tract of both humans and animals. P. mirabilis can cause a wide variety of infections, however it does not harbor any intrinsic β-lactamase genes and as such usually exhibits full susceptibility to β-lactams with the exception of imipenem, to which it is naturally resistant. OXA-23 enzymes are carbapenem-hydrolyzing class D β-lactamases and are usually the main cause of acquired resistance to carbapenems in Acinetobacter baumannii but have recently been reported in P. mirabilis in France. In this report we describe the emergence of OXA-23-producing P. mirabilis clinical isolates from Switzerland.