Free-standing serum-albumin mats can transport protons over millimetre length-scales. The results of photoinduced proton transfer and voltage-driven proton conductivity measurements, together with temperature dependent and isotope effect studies, suggest that oxo-amino-acids of the protein serum albumin play a major role in the translocation of protons via an “over-the-barrier” hopping mechanism. The use of proton-conducting protein mats opens new possibilities for bioelectronic interfaces.