Significance Some peptide toxins act by stabilizing the voltage sensor domains (VSDs) of voltage-gated channels in open or closed conformations. hHv1 is a human voltage-gated proton channel and has lacked a specific inhibitor to assess its roles in physiology. We designed a phage-display library of 1 million novel peptides sharing an inhibitor cysteine knot (ICK) scaffold by combining elements of natural toxins; phagemids expressing Corza6 (C6) were selected by their capacity to bind to hHv1 protein. Two C6 peptides bind to the two VSDs in hHv1 and thereby inhibit channel operation. C6 was employed to confirm hypothesized roles for hHv1 in human sperm and white blood cells. The method should prove amenable to designer toxin identification for other voltage-gated channels.