Efficient and sustainable ultraviolet (UV)‐blocking materials are of great interest in many fields. Herein, novel cellulose‐based UV‐blocking films are developed via surface modification using the Biginelli reaction. The resulting films exhibited excellent visible transparency (80%) at 550 nm and superhigh UV‐blocking performance, which can shield almost 100% UVA and UVB. These features are very stable even the materials are being subjected to solvents, UV irradiation, and thermal treatment. This work provides a novel and facile strategy to fabricate functional cellulose‐based films with superhigh anti‐ultraviolet performance. [ABSTRACT FROM AUTHOR]