Pathogens have co-evolved with mosquitoes to optimize transmission to hosts. Mosquito salivary-gland extract is known to modulate host immune responses and facilitate pathogen transmission, but the underlying molecular mechanisms of this have remained unknown. In this study, we identified and characterized a prominent 15-kilodalton protein, LTRIN, obtained from the salivary glands of the mosquito Aedes aegypti. LTRIN expression was upregulated in blood-fed mosquitoes, and LTRIN facilitated the transmission of Zika virus (ZIKV) and exacerbated its pathogenicity by interfering with signaling through the lymphotoxin-β receptor (LTβR). Mechanically, LTRIN bound to LTβR and 'preferentially' inhibited signaling via the transcription factor NF-κB and the production of inflammatory cytokines by interfering with the dimerization of LTβR during infection with ZIKV. Furthermore, treatment with antibody to LTRIN inhibited mosquito-mediated infection with ZIKV, and abolishing LTβR potentiated the infectivity of ZIKV both in vitro and in vivo. This study provides deeper insight into the transmission of mosquito-borne diseases in nature and supports the therapeutic potential of inhibiting the action of LTRIN to disrupt ZIKV transmission.