A theory that predicts the rate of a proton-coupled electron transfer reaction is presented. As a specific application, we consider an electron donor and acceptor separated by a hydrogen-bonded interface that may undergo a proton transfer reaction. The proton transfer reaction occurs in an asymmetric interface with a significant charge displacement upon proton transfer. Thus, the proton transfer is driven by solvent polarization fluctuations in a similar fashion to the electron transfer. The electron and proton may transfer consecutively-electron transfer (ET) followed by proton transfer (PT)-as two separate tunnel reactions (ET/PT), or they may transfer concertedly-electron and proton transfer in one tunnel event (ETPT). We obtain an expression for the rate of the concerted process based on an analysis of the tunneling path in the two-dimensional electron-proton coordinate space. The rates for ETPT and ET/PT are evaluated for a model reaction complex that mimics an electron donor-hydrogen-bonded interface-electron acceptor system. The parameters governing the rates for these different channels are evaluated for the model, and this permits prediction of which reaction channel, ET/PT or ETPT, will dominate.