During herbivore feeding, plants can recognize herbivore-associated molecular patterns (HAMPs) present in saliva and trigger pattern-triggered immunity (PTI). Piercing-sucking insects secrete gel saliva, forming salivary sheaths that aid in feeding. However, the role of proteins within these salivary sheaths in modulating plant defences remains poorly understood. In this study, we identified a novel HAMP, Nlsp5, from the salivary sheath of the brown planthopper (Nilaparvata lugens, BPH). Nlsp5 is a planthopper-specific protein and acts as an elicitor of BAK1-dependent PTI responses in both tobacco and rice plants. Moreover, the 19-amino-acid peptide (NP19) within Nlsp5 functions as a minimal immunogenic epitope, which is specifically recognized by plants, stimulating jasmonic acid and hydrogen peroxide pathways. Through exogenous treatment with synthetic NP19 and overexpressing Nlsp5 in rice, we further found that the induced defence responses not only impaired planthopper performance directly but also triggered the emission of volatile compounds that attract a common parasitoid. Additionally, NP19 treatment enhanced the resistance of rice, tobacco, and cotton to several chewing and sap-sucking insects. However, silencing Nlsp5 in BPH disrupted salivary sheath formation, reducing insect feeding efficiency. This study demonstrates that Nlsp5 from the BPH salivary sheath acts as an unavoidable HAMP, triggering resistance in multiple plants to various insect pests. The critical role of this protein in insect feeding precludes evolutionary adaptations to evade detection by plants.