Abstract: As a critical component of batteries, traditional commercial polyolefin separators suffer from inherent drawbacks, such as poor thermal stability and low electrolyte wettability. Consequently, the development of green, high-temperature resistant, and high electrolyte-affinitive cellulose-based separators has become a research hotspot. Compared with the well-established the relatively mature research and technological foundation of cellulose-based separators for lithium-ion batteries, the development for sodium-ion batteries is still in its infancy. To identify future development paths for sodium-ion battery separators, this paper provides a detailed review of three major material systems: natural cellulose, cellulose derivatives, and nanocellulose. It analyzes the advantages and disadvantages of four preparation processes—vacuum filtration, non-solvent induced phase separation, electrospinning, and freeze-drying—while examining their impacts on the pore structure of separators. Furthermore, this study discusses research progress in enhancing mechanical strength and ion transport through strategies such as nanofiller compounding and surface coating. Finally, the prospects for cellulose-based separators in sodium-ion batteries are discussed in light of current research trends.