We have previously identified neutralizing human monoclonal antibodies against Nipah virus (NiV) and Hendra virus (HeV) by panning a large nonimmune antibody library against a soluble form of the HeV attachment-envelope glycoprotein G (sG HeV). One of these antibodies, m102, which exhibited the highest level of cross-reactive neutralization of both NiV and HeV G, was affinity maturated by light-chain shuffling combined with random mutagenesis of its heavy-chain variable domain and panning against sGHeV. One of the selected antibody Fab clones, m102.4, had affinity of binding to sGHeV that was equal to or higher than that of the other Fabs; it was converted to IgG1 and tested against infectious NiV and HeV. It exhibited exceptionally potent and cross-reactive inhibitory activity with 50% inhibitory concentrations below 0.04 and 0.6 microg/mL, respectively. The virus-neutralizing activity correlated with the binding affinity of the antibody to sG HeV and sG NiV. m102.4 bound a soluble form of NiV G (sG NiV) better than it bound sG HeV, and it neutralized NiV better than HeV, despite being originally selected against sG HeV. These results suggest that m102.4 has potential as a therapeutic agent for the treatment of diseases caused by henipaviruses. It could be also used for prophylaxis and diagnosis, and as a research reagent.