The implementation of three-dimensional electrodes in electrochemical reactors provides an effective method to improve the efficiency of some electrochemical processes, such as wastewater treatment, electrosynthesis, and energy storage. The increase of the surface area due to the high porosity of these materials is of special interest in diffusion-convection controlled processes. The flow distribution inside the reactor compartments is highly influenced by the characteristics of the porous material. The hydrodynamic behavior of a filter-press electrochemical reactor assembled with three-dimensional electrodes made of carbon felt has been studied in this paper. The felt thickness/cell thickness ratio has been optimized on the basis of flow distribution and energetic considerations, such as pumping power and electrode-collector contact losses.