The development of facile, rapid, low-energy, environmentally benign routes for the synthesis of covalent organic frameworks (COFs) is of great interest. This study concerns the utilization of water containing dissolved CO2 as a solvent for the room-temperature synthesis of COF. The as-synthesized particles, denoted COF-LZU1, combine advantages of good crystallinity, nanoscale size, and high surface area, which suggests promising application as a support for heterogeneous catalysts. Moreover, this versatile CO2-assisted method is also applicable for the room-temperature synthesis of Cu-COF-LZU1. This method gives rise to new opportunities for fabricating COFs and COF-based materials with different compositions and structures.