Nanofluid reverse electrodialysis (RED) is considered the most promising technology for harvesting osmotic energy. As materials used for RED, natural nanofluid materials are renewable but suffer from low power densities due to weak surface charge densities or irregular ion transport channels, while advanced materials from sophisticated manufacturing are too expensive. Here, a pair of novel natural nanofluid membranes with channel-like nanopores for highly improved selective ion transport were developed by assembling negatively and positively charged bacterial cellulose nanofibers via a space-confined flattened extrusion process. The regular internal channel-like nanopores with strongly increased surface charges assured less electrical imperfectness and therefore high ion selectivity, leading to RED systems with a high output power density of 0.72 W m−2 (5.58 W m−2 of negatively charged membranes), far superior to other existing natural nanofluidic RED systems. This strategy of efficiently enhancing the selective ion transport will open up a new route for the development of nanofluidic RED devices, and allows a wide range of their applications in other energy fields.