The conformational rearrangement of N- and C-heptad repeats (NHR, CHR) of the HIV-1 glycoprotein-41 (gp41) ectodomain into a trimer of hairpins triggers virus-cell fusion by bringing together membrane-spanning N- and C-terminal domains. Peptides derived from the NHR and CHR inhibit fusion by targeting a prehairpin intermediate state of gp41. Typically, peptides derived from the CHR are low nanomolar inhibitors, whereas peptides derived from the NHR are low micromolar inhibitors. Here, we describe the inhibitory activity of swapped-domain gp41 mimics of the form CHR-loop-NHR, which were designed to form reverse hairpin trimers exposing NHR grooves. We observed low nanomolar inhibition of HIV fusion in constructs that possessed the following properties: an extended NHR C-terminus, an exposed conserved hydrophobic pocket on the NHR, high helical content, and trimer stability. Low nanomolar activity was independent of CHR length. CD studies in membrane mimetic dodecylphosphocholine micelles suggested that bioactivity could be related to the ability of the inhibitors to interact with a membrane-associated prehairpin intermediate. The swapped-domain design resolves the problem of unstable and weakly active NHR peptides and suggests a different mechanism of action from that of CHR peptides in inhibition of HIV-1 fusion.