The Arabidopsis immune receptor FLS2 and co-receptor BAK1 perceive the bacterial flagellin epitope flg22 to activate plant immunity. To prevent this response, phytopathogenic bacteria deploy a repertoire of effector proteins to perturb immune signaling. However, the effector-induced perturbation is often sensed by the host, triggering another layer of immunity. We report that the Pseudomonas syringae effector HopB1 acts as a protease to cleave immune-activated BAK1. Prior to activation, HopB1 constitutively interacts with FLS2. Upon activation by flg22, BAK1 is recruited to the FLS2-HopB1 complex and is phosphorylated at Thr455. HopB1 then specifically cleaves BAK1 between Arg297 and Gly298 to inhibit FLS2 signaling. Although perturbation of BAK1 is known to trigger increased immune responses in plants, the HopB1-mediated cleavage of BAK1 leads to enhanced virulence, but not disease resistance. This study thus reveals a virulence strategy by which a pathogen effector attacks the plant immune system with minimal host perturbation.