Biological passivation of porous silicon (PSi)‐based optical devices is demonstrated. Infiltration by small amphiphilic fungal proteins called HFBs changes the wettability of the PSi surface (see figure) and protects the sponge‐like structure against dissolution by basic solutions. This protein membrane leaves the sensing ability of an optical transducer unchanged, adding chemical stability that can be the key in biomolecular experiments.