Protein nanolayers (PNLs) formed at an electrochemical liquid|liquid interface between water (W) and a fluorous solvent (F) were examined by using interfacial rheological measurement (IRM) and neutron reflectometry (NR) under the externally controlled condition of the phase boundary potential differences (= φ - φ + const.), where F contained a hydrophobic ionic liquid (IL) as a supporting electrolyte and W, whose pH was 7.4, contained a protein, bovine serum albumin (BSA). The IRM and NR results illuminated that both static and dynamic properties of the PNL at the electrochemical F|W interface were varied by applying . NR found minimal dependence on the adsorption amount of BSA in the PNL. In contrast, IRM revealed that although the interfacial shear loss moduli ″ of the PNL was constant regardless of , the interfacial shear storage ' of the PNL increased dramatically at more negative , showing a more elastic response. This difference between static and dynamic properties results from the increase in intermolecular and intramolecular interactions between BSA molecules in the PNL at more negative due to the accelerated denaturation of negatively charged BSA that formed complexes with IL cations accumulated on the F side of the F|W interface. The ' and ″ reversibly responded to switching between different potentials (a positive and a negative ). These IRM results unveiled that the viscoelasticity of the PNL at the electrochemical F|W interface is reversibly potential-switchable. The present interface-specific method using the potential control is a new promising method to diversify and switch the PNL structure reversibly. The reversible structural control of the PNL would enable us to perform real-time observation of cells reacting to environmental changes at liquid|liquid interfaces.