Reverse electrodialysis (RED) is known as an efficient way of converting the salinity gradient between river water and sea water into energy. However, the high cost and complex fabrication of the necessary ion exchange membranes greatly prohibit the development of the RED process. For the first time, an ionized wood membrane is demonstrated for this application, benefiting from the advantages of natural wood, which is abundant, low cost, sustainable, and easy to scale. The wood membrane maintains the aligned nanochannels of the cellulose nanofibers derived from the natural wood. The surface of the nanochannels can be functionalized to positively or negatively charged by in situ modifying the hydroxyl groups on the cellulose chains to quaternary ammonium or carboxyl groups, respectively. These charged aligned nanochannels serve as nanofluidic passages for selective ion transport with opposite polarity through the wood membrane, resulting in efficient charge separation and generating an electrochemical potential difference. The all‐wood RED device with 100 cells using a scalable stacking geometry generates an output voltage as high as 9.8 V at open circuit from a system of synthetic river water and sea water.