The growing environmental concern over petrochemical‐based plastics continuously promotes the exploration of green and sustainable substitute materials. Compared with petrochemical products, cellulose has overwhelming superiority in terms of availability, cost, and biodegradability; however, cellulose's dense hydrogen‐bonding network and highly ordered crystalline structure make it hard to be thermoformed. A strategy to realize the partial disassociation of hydrogen bonds in cellulose and the reassembly of cellulose chains via constructing a dynamic covalent network, thereby endowing cellulose with thermal processability as indicated by the observation of a moderate glass transition temperature (Tg = 240 °C), is proposed. Moreover, the cellulosic bioplastic delivers a high tensile strength of 67 MPa, as well as excellent moisture and solvent resistance, good recyclability, and biodegradability in nature. With these advantageous features, the developed cellulosic bioplastic represents a promising alternative to traditional plastics. A “dynamic covalent network” reconstruction strategy is proposed for the fabrication of sustainable cellulosic bioplastics. By introducing dynamic linkages between cellulose chains, this cellulosic bioplastic is imparted with good thermo‐processability, competitive mechanical properties, excellent water and solvent resistance, as well as chemical and biological degradability. This approach provides a new route for developing sustainable and degradable bioplastics from resource‐abundant biomass.