We present a method for the renaturation of denatured cytochrome c in a phosphonate-type ionic liquid with the addition of a suitable zwitterion and a small amount of water. The polarity of zwitterions was found to be a critical property for the renaturation of cytochrome c. The renatured cytochrome c was reduced by the addition of sodium dithionite, suggesting that the renatured cytochrome c retains its redox ability.