Pathogen-associated molecular patterns (PAMPs) elicit basal defense responses in plants, and, in turn, pathogens have evolved mechanisms to overcome these PAMP-induced defenses. To suppress immunity, the phytopathogenic bacterium Pseudomonas syringae secretes effector proteins, the biochemical function and virulence targets of which remain largely unknown. We show that HopAI1, an effector widely conserved in both plant and animal bacterial pathogens, inhibits the Arabidopsis mitogen-activated protein kinases (MAPKs) activated by exposure to PAMPs. HopAI1 inactivates MAPKs by removing the phosphate group from phosphothreonine through a unique phosphothreonine lyase activity, which is required for HopAI1 function. The inhibition of MAPKs by HopA1 suppresses two independent downstream events, namely the reinforcement of cell wall defense and transcriptional activation of PAMP response genes. The MAPKs MPK3 and MPK6 physically interact with HopAI1 indicating that they are direct targets of HopAI1. These findings uncover a mechanism by which Pseudomonas syringae overcomes host innate immunity to promote pathogenesis.