Pathogens have co-evolved with mosquitoes to optimize transmission to hosts. Mosquito salivary-gland extract is knownto modulate host immune responses and facilitate pathogen transmission, but the underlying molecular mechanisms of thishave remained unknown. In this study, we identified and characterized a prominent 15-kilodalton protein, LTRIN, obtainedfrom the salivary glands of the mosquito Aedes aegypti. LTRIN expression was upregulated in blood-fed mosquitoes, and LTRINfacilitated the transmission of Zika virus (ZIKV) and exacerbated its pathogenicity by interfering with signaling through thelymphotoxin-β receptor (LTβ R). Mechanically, LTRIN bound to LTβ R and ‘preferentially’ inhibited signaling via the transcrip-tion factor NF-κ B and the production of inflammatory cytokines by interfering with the dimerization of LTβ R during infectionwith ZIKV. Furthermore, treatment with antibody to LTRIN inhibited mosquito-mediated infection with ZIKV, and abolishingLTβ R potentiated the infectivity of ZIKV both in vitro and in vivo. This study provides deeper insight into the transmission ofmosquito-borne diseases in nature and supports the therapeutic potential of inhibiting the action of LTRIN to disrupt ZIKVtransmission.