Xanthomonas oryzae delivers effector proteins into host cells through a type III secretion system to inhibit host immune responses, but how these effectors suppress host immunity is largely unknown. Here we found that Xoo2875, one of the effectors of X. oryzae, strongly inhibited host resistance to X. oryzae. Transgenic rice plants expressing Xoo2875 exhibited semi-dwarfism and a reduction in Brassinolide-dependent laminar inclination, characteristics of brassinosteroid (BR)-insensitive mutants caused by mutations of the BR receptor. A yeast two-hybrid experiment indicated that Xoo2875 interacted with OsBAK1, an essential component of both microbe-associated molecular patterns (MAMPs) and BR receptors, suggesting that the virulent activity of Xoo2875 is mediated by inhibition of OsBAK1. Expression of Xoo2875 in Arabidopsis cells activated host immune responses, suggesting the presence of intracellular immune receptors that recognize Xoo2875. Because Xoo2875 homologs are highly conserved in Xanthomonas species, the development of Xoo2875-induced immunity is probably a useful strategy to avoid pathogen invasion.