Saliva plays a crucial role in mediating plant–insect interactions, yet the functional diversity of salivary proteins remains poorly understood. Here, we identify NlSP6935, a salivary gland-specific protein conserved among rice planthoppers but absent in bamboo-feeding relatives. Silencing NlSP6935 causes severe lethality, feeding impairment, and infertility in Nilaparvata lugens, independent of host plant resistance. Transient expression assays reveal that NlSP6935 suppresses H₂O₂ accumulation in plants, while overexpression in rice downregulates terpenoid biosynthesis and enhances host attractiveness. However, transgenic NlSP6935 plants only weakly rescue RNAi-induced lethality, demonstrating its dual role in insect physiology and plant defense suppression. Our findings reveal a novel effector essential for both planthopper survival and host adaptation, providing new insights into pest control strategies.