Cell-secreted flavin binds to outer-membrane c-type cytochromes (OM c-Cyts) as a redox cofactor in Shewanella oneidensis MR-1, generating a semiquinone (Sq) state to enhance the rate of extracellular electron-transport (EET) process by several orders of magnitude. Here, as ionic strength (Is) is a major factor in stabilizing bound Sq in flavoproteins, we examined the influence of Is on the flavin affinity in OM c-Cyts to promote Sq formation for enhancing the rate of the EET process. Estimated dissociation constants showed that an increase in Is induces threefold higher Sq formation in OM c-Cyts. However, the higher Is neither resulted in the larger current production nor current enhancement by flavin addition. Strong Is dependency for the redox potential of heme centers in OM c-Cyts suggests that Is not only controls the stability of Sq, but also alters coupling constants among redox centers in OM c-Cyts through structural changes. Outside the box: The effect of ionic strength on the increase in thermodynamic stability of a bound-flavin cofactor in outer-membrane c-type cytochromes in explored to enhance the rate of the extracellular electron-transport process in Shewanella oneidensis MR-1.