SignificanceSuperantigens are proteins that induce a vast activation of the immune system and are typically found in pathogenic microbes. Recently, proteins with superantigen-like properties were discovered in a commensal bacterium inhabiting the human gut, but little is known about the impact of this protein on host/commensal interaction. Here, we characterize the superantigen-like protein immunoglobulin-binding protein (Ibp) expressed byRuminococcus gnavus, which has broad specificity to human and mouse immunoglobulins. We present the crystal structure of the Ibp/Fab complex and show that one of Ibp’s domains alone is sufficient to activate B cells. This work provides an understanding of superantigen-like molecules potentially playing a role in the maintenance of host/microbiome homeostasis.