Pathogen effector proteins are delivered to host cells to suppress plant immunity. However, the mechanisms by which effector proteins function are largely unknown. Here we show that expression of XopP(Xoo), an effector of rice pathogen Xanthomonas oryzae pv. oryzae, in rice strongly suppresses peptidoglycan (PGN)- and chitin-triggered immunity and resistance to X. oryzae. XopP(Xoo) targets OsPUB44, a rice ubiquitin E3 ligase with a unique U-box domain. We find that XopP(Xoo) directly interacts with the OsPUB44 U-box domain and inhibits ligase activity. Two amino-acid residues specific for the OsPUB44 U-box domain are identified, which are responsible for the interaction with XopP(Xoo). Silencing of OsPUB44 suppresses PGN- and chitin-triggered immunity and X. oryzae resistance, indicating that OsPUB44 positively regulates immune responses. Thus, it is likely that XopP(Xoo) suppresses immune responses by directly interacting with and inhibiting a positive regulator of plant immunity.