The cathode catalyst layer in a proton exchange membrane fuel cell is now known to contain ionomer nanofibers and experiments have demonstrated a fuel cell performance increase of ∼10% due to those nanofibers. The experiments demonstrate that ionomer nanofibers have proton conductivities that exceed those of the bulk form of the ionomer by more than an order of magnitude. A new model of a proton exchange membrane fuel cell is presented here that predicts the effect of nanofibers on cell performance in terms of the enhanced nanofiber proton conductivity and other relevant variables. The model peak cell power density is ∼7% greater for the case with 10% of the cathode catalyst layer ionomer in nanofiber form versus the same case without nanofibers. This difference is consistent with trends observed in previously published experimental results. These results are significant since they suggest alternative methods to reduce platinum loading in fuel cells and to optimize fuel cell performance.