Ionic transistors from organic and biological materials represent an emerging class of devices for bioelectronics applications. Within this context, protonic transistors are exciting targets for further research and development. Indeed, given the ubiquity of proton transport and transfer phenomena, protonic devices represent a natural choice for interfacing rugged traditional electronics and biological systems, facilitating the sensitive transduction of biochemical events into electrical signals. Recently, we have discovered unexpected protonic conductivity in the cephalopod structural protein reflectin. We have characterized this material with a diverse array of electrochemical techniques, finding that its electrical figures of merit compare favorably to those of artificial proton conductors. Such favorable electrical properties have made it possible to use reflectin in protein-based protonic transistors. Overall, our findings may hold significance for a broad range of biomedical and bioelectrochemical devices from naturally occurring materials.