Structural determination of the salicylic-acid-binding core ofArabidopsisNPR4 sheds light on the mechanisms through which this plant hormone interacts with its receptors, providing insights that are of potential use in engineering enhanced immunity. Salicylic acid (SA) is a plant hormone that is critical for resistance to pathogens(1-3). The NPR proteins have previously been identified as SA receptors(4-10), although how they perceive SA and coordinate hormonal signalling remain unknown. Here we report the mapping of the SA-binding core ofArabidopsis thalianaNPR4 and its ligand-bound crystal structure. The SA-binding core domain of NPR4 refolded with SA adopts an alpha-helical fold that completely buries SA in its hydrophobic core. The lack of a ligand-entry pathway suggests that SA binding involves a major conformational remodelling of the SA-binding core of NPR4, which we validated using hydrogen-deuterium-exchange mass spectrometry analysis of the full-length protein and through SA-induced disruption of interactions between NPR1 and NPR4. We show that, despite the two proteins sharing nearly identical hormone-binding residues, NPR1 displays minimal SA-binding activity compared to NPR4. We further identify two surface residues of the SA-binding core, the mutation of which can alter the SA-binding ability of NPR4 and its interaction with NPR1. We also demonstrate that expressing a variant of NPR4 that is hypersensitive to SA could enhance SA-mediated basal immunity without compromising effector-triggered immunity, because the ability of this variant to re-associate with NPR1 at high levels of SA remains intact. By revealing the structural mechanisms of SA perception by NPR proteins, our work paves the way for future investigation of the specific roles of these proteins in SA signalling and their potential for engineering plant immunity.