Streptococcus suis serotype 2 is a highly invasive, extracellular pathogen in pigs with the capacity to cause severe infections in humans. This study was initiated by the finding that IgM degradation products are released after opsonization of S. suis. The objective of this work was to identify the bacterial factor responsible for IgM degradation. The results of this study showed that a member of the IdeS family, designated Ide(Ssuis) (Immunoglobulin M-degrading enzyme of S. suis), is responsible and sufficient for IgM cleavage. Recombinant Ide(Ssuis) was found to degrade only IgM but neither IgG nor IgA. Interestingly, Western blot analysis revealed that Ide(Ssuis) is host specific, as it exclusively cleaves porcine IgM but not IgM from six other species, including a closely related member of the Suidae family. As demonstrated by flow cytometry and immunofluorescence microscopy, Ide(Ssuis) modulates binding of IgM to the bacterial surface. Ide(Ssuis) is the first prokaryotic IgM-specific protease described, indicating that this enzyme is involved in a so-far-unknown mechanism of host-pathogen interaction at an early stage of the host immune response. Furthermore, cleavage of porcine IgM by Ide(Ssuis) is the first identified phenotype reflecting functional adaptation of S. suis to pigs as the main host.