Well ordered SBA-15 mesoporous silicas are prepared using a simple room temperature procedure made possible by the addition of small amounts of cellulose nanoparticles to a synthesis gel containing Si, water, ethanol, HCl, and P123 co-polymer. The presence of the cellulose nanoparticles enhances the level of silica condensation, in addition to improving the mesostructural ordering and results in SBA-15 materials with very low microporosity. The cellulose acts by reducing the solubility of the P123 co-polymer and also segregates the hydrophilic ethylene oxide segments of the co-polymer from the forming silica framework. These actions are conducive for the formation and phase separation of the P123–silica mesophase. Small amounts of cellulose-related organics are retained in the SBA-15 mesophase, although the pores are templated by P123 co-polymer.