The linear pentadecapeptide gramicidin A forms an ion channel in the lipid bilayer to selectively transport monovalent cations. Nevertheless, we have surprisingly found that gramicidin A directly inhibits mammalian Na+/K+-ATPase. Gramicidin A inhibited ATP hydrolysis by Na+/K+-ATPase from porcine cerebral cortex at the IC50 value of 8.1 μM, while gramicidin S was approximately fivefold less active. The synthetic gramicidin A analog lacking N-terminal formylation and C-terminal ethanolamine exhibited a weaker inhibitory effect on the ATP-hydrolyzing activity of Na+/K+-ATPase than gramicidin A, indicating that these end modifications are necessary for gramicidin A to inhibit Na+/K+-ATPase activity. Moreover, Lineweaver–Burk analysis showed that gramicidin A exhibits a mixed type of inhibition. In addition to the most well-studied ionophore activity, our present study has disclosed a novel biological function of gramicidin A as a direct inhibitor of mammalian Na+/K+-ATPase activity.