In this work, exfoliated vermiculite sheets readily formed transparent, flexible membranes having a layered microstructure by intercalation composite method with cellulose nanofibers. The resulting vermiculite-cellulose nanofiber (VMT-CNFs) composite membrane exhibited uniform interlayer spacing due to electrostatic interaction between the nanofibers and the vermiculite sheets. The proton conductivity of the composite membrane was enhanced compared to the pure cellulose nanofiber membrane. This was because the interlayer space in the composite membrane formed hydrophilic nanochannels for proton transport as the two components contain a large number of hydroxyl groups. In addition, due to the high-temperature decomposition resistance of vermiculite crystals, the thermal stability of composite membranes was significantly improved upon the addition of vermiculite sheets. The high-temperature proton conduction of VMT-CNFs composite membrane (100 degrees C, 0.043 Scm(-1)) was superior to that of the pure cellulose nanofiber membrane (100 degrees C, 0.024 Scm(-1)). Therefore, these composite membranes are suitable for proton conduction applications at high temperatures.