Ibalizumab is a humanized monoclonal antibody that binds human CD4—a key receptor for HIV—and blocks HIV-1 infection. However, HIV-1 strains with mutations resulting in loss of an N-linked glycan from the V5 loop of the envelope glycoprotein gp120 are resistant to ibalizumab. Previous structural analysis suggests that this glycan fills a void between the gp120 V5 loop and the ibalizumab light chain, perhaps causing steric hindrance that disrupts viral entry. If this void contributes to HIV-1 resistance to ibalizumab, we reasoned that 'refilling' it by engineering an N-linked glycan into the ibalizumab light chain at a position spatially proximal to gp120 V5 may restore susceptibility to ibalizumab. Indeed, one such ibalizumab variant neutralized 100% of 118 diverse HIV-1 strains tested in vitro, including 10 strains resistant to parental ibalizumab. These findings demonstrate that the strategic placement of a glycan in the variable region of a monoclonal antibody can substantially enhance its activity.