Evidence suggests the bactericidal activity of mitochondria-derived reactive oxygen species (mROS) directly contributes to killing phagocytozed bacteria. Infection-responsive components that regulate this process remain incompletely understood. We describe a role for the mitochondria-localizing enzyme encoded by Immunoresponsive gene 1 (IRG1) during the utilization of fatty acids as a fuel for oxidative phosphorylation (OXPHOS) and associated mROS production. In a zebrafish infection model, infection-responsive expression of zebrafish irg1 is specific to macrophage-lineage cells and is regulated cooperatively by glucocorticoid and JAK/STAT signaling pathways. Irg1-depleted macrophage-lineage cells are impaired in their ability to utilize fatty acids as an energy substrate for OXPHOS-derived mROS production resulting in defective bactericidal activity. Additionally, the requirement for fatty acid β-oxidation during infection-responsive mROS production and bactericidal activity toward intracellular bacteria is conserved in murine macrophages. These results reveal IRG1 as a key component of the immunometabolism axis, connecting infection, cellular metabolism, and macrophage effector function.