Neisseria gonorrhoeae deploys a unique immune evasion strategy wherein it caps its lipooligosaccharide (LOS) with and host-derived CMP-sialic acid (CMP-N-acetylneuraminic acid in humans) using its LOS sialyltransferase (Lst), an enzyme that is highly conserved across N. gonorrhoeae and Neisseria meningitidis. LOS sialylation enables gonococci to evade complement and cathelicidins such as LL-37, while down-regulating inflammatory responses by engaging Siglec receptors. Female BALB/c mice were immunized intramuscularly with recombinant gonococcal Lst adjuvanted with monophosphoryl lipid A (MPL) plus Alum and challenged 2 weeks post-immunization intravaginally with N. gonorrhoeae FA1090. Compared to adjuvant controls, Lst-immunized mice cleared gonococci significantly faster (median time to clearance 3 d vs 7 d, P=0.0009) and showed reduced burden of infection (~80% reduction in Area Under Curve (AUC), P=0.003). Consistent with LOS sialylation that occurs in vivo, gonococci recovered from the vaginas of adjuvant control mice were fully resistant to killing (>100% survival) by normal human serum (NHS). By contrast, gonococci recovered from Lst-immunized mice were killed by NHS (33% survival; P=0.016 compared to adjuvant controls). Immune IgG from Lst-immunized mice, but not adjuvant control IgG, prevented gonococci grown in media containing CMP-sialic acid from becoming resistant to NHS, suggesting that anti-Lst antibody blocks Lst function. Gonococci evade complement in a human-specific manner because they bind only human complement inhibitors such as Factor H (FH) and C4b-binding protein (C4BP). Using transgenic mice that expressed human FH and C4BP, we showed that the Lst vaccine reduced median time to clearance (4 d vs 8 d (P=0.002)) and bacterial burden (73% reduction in AUC (P=0.03)) compared to adjuvant controls, thereby maintaining activity even in the presence of human complement regulatory mechanisms. Terminal complement was required for protection, evidenced by loss of efficacy in mice treated with a function-blocking anti-C7 monoclonal antibody (mAb). We generated a panel of unique anti-Lst mAbs, all of which blocked LOS sialylation-mediated serum resistance. A combination of anti-Lst mAbs given intravenously one day before infection also reduced the duration and burden of gonococcal vaginal colonization in mice. Lst, a key gonococcal virulence factor, is an attractive target for gonococcal vaccines and preventive mAbs.