Pathogenic bacteria use the type III secretion system to deliver effector proteins into host cells to modulate the host signaling pathways. In this study, the Shigella type III effector OspF was shown to inactivate mitogen-activated protein kinases (MAPKs) [extracellular signal-regulated kinases 1 and 2 (Erk1/2), c-Jun N-terminal kinase, and p38]. OspF irreversibly removed phosphate groups from the phosphothreonine but not from the phosphotyrosine residue in the activation loop of MAPKs. Mass spectrometry revealed a mass loss of 98 daltons in p-Erk2, due to the abstraction of the α proton concomitant with cleavage of the C-OP bond in the phosphothreonine residue. This unexpected enzymatic activity, termed phosphothreonine lyase, appeared specific for MAPKs and was shared by other OspF family members.