Significance Inflammation is an important arm of host defense against microbial infections, but excessive inflammation can cause human diseases. Interferon regulatory factor 5 (IRF5) is a key regulator of inflammatory responses, controlling the expression of many proinflammatory cytokines. Mutations and dysregulation of IRF5 have been linked to autoimmune and autoinflammatory diseases in humans; however, how IRF5 is activated is not well understood. We report that the kinase IKKβ, which is known to regulate the Rel/nuclear factor kappa B (NF-κB) family of transcription factors, phosphorylates IRF5 at a specific serine residue, and that this phosphorylation is critical for IRF5 activation and cytokine production. Thus, IKKβ regulates two master transcription factors, NF-κB and IRF5, which coordinately control gene expression to mediate inflammatory responses.