Metacaspases are distant relatives of animal caspases found in protozoa, fungi, and plants. Limited experimental data exist defining their function(s), despite their discovery by homology modeling a decade ago. We demonstrated that two type I metacaspases, AtMC1 and AtMC2, antagonistically control programmed cell death in Arabidopsis. AtMC1 is a positive regulator of cell death and requires conserved caspase-like putative catalytic residues for its function. AtMC2 negatively regulates cell death. This function is independent of the putative catalytic residues. Manipulation of the Arabidopsis type I metacaspase regulatory module can nearly eliminate the hypersensitive cell death response (HR) activated by plant intracellular immune receptors. This does not lead to enhanced pathogen proliferation, decoupling HR from restriction of pathogen growth.