Significance Under anoxic conditions, various microorganisms couple the oxidation of organic carbon to the reduction of solid ferric iron oxide phases using extracellular electron shuttles (EES). Determining the contribution of this widespread terminal electron accepting process to total anaerobic respiration has proven challenging because of large variations in observed ferric iron reduction rates. This study demonstrates that rates of ferric iron oxide reduction by EES can be rationalized based on a unifying relationship that links rates to the thermodynamic driving force for the least favorable electron transfer from the EES to ferric iron. The relationship derived herein allows for a generalized and precise assessment of the contribution of EES-facilitated ferric iron oxide reduction to organic matter decomposition in anoxic environments.