The voltage-gated Kv1.3 channel in T lymphocytes is a validated therapeutic target for diverse autoimmune diseases. Here we review the discovery of Kv1.3, its physiological role in T cells, and why it is an attractive target for modulating autoimmune responses. We focus on peptide inhibitors because the first Kv1.3-selective inhibitor in human trials is a peptide derived from a marine organism. Two broad classes of peptides block Kv1.3, the first from scorpions and the second from sea anemones. We describe their structures, their binding site in the external vestibule of Kv1.3, how they have been engineered to improve Kv1.3-specificity, and their pharmacokinetic and pharmacodynamic properties. Finally, we highlight the therapeutic potential of Kv1.3 peptide inhibitors to treat autoimmune diseases without compromising protective immune responses.