Aim: To investigate the biochemical and functional properties of an extracellular protease, RpA, in Ralstonia pickettii WP1 isolated from water supply systems. Methods and Results: An extracellular protease was identified and characterized from R. pickettii WP1. A mutant strain WP1M2 was created from strain WP1 by mini-Tn5 transposition. The culture filtrates from WP1M2 had a lower cytotoxic effect than the parental WP1 on several mammalian cell lines. Cloning and sequence analysis revealed the Tn5 transposon inserted at a protease gene (rpA) which is 81% homologous to prtA and aprX genes of Pseudomonas fluorescens. The rpA gene encodes a 482-residue protein showing sequence similarity to metalloproteases of the serralysin family. The RpA protein was expressed in Escherichia coli using a pET expression vector and purified as a 55 kDa molecular weight protein. Furthermore, the protease activity of RpA was inhibited by protease inhibitor and heat treatment. Conclusions: The in vitro cytotoxic activity of R. pickettii culture filtrates was attributed to RpA protease. Significance and Impact of the Study: An extracellular protease, RpA, was identified from R. pickettii WP1 isolated from water supply system. The RpA metalloproteases is required for the pathogenicity of R. pickettii to mammalian cell lines.