Enzymatic synthesis of cellulose and its derivatives has gained considerable attention for use in the production of artificial crystalline nanocelluloses with unique structural and functional properties. However, the poor colloidal stability of the nanocelluloses during enzymatic synthesis in aqueous solutions limits their crystallization-based self-assembly to greater architectures. In this study, oligo(ethylene glycol) (OEG)-bearing cellulose oligomers with different OEG chain lengths were systematically synthesized via cellodextrin phosphorylase-catalyzed oligomerization of α-d-glucose l-phosphate monomers against OEG-bearing β-d-glucose primers. The products were self-assembled into extremely well-grown crystalline nanoribbon network structures with the cellulose II allomorph, potentially due to OEG-derived colloidal stability of the nanoribbon's precursors, followed by the in situ formation of physically cross-linked hydrogels. The monomer conversions, average degree of polymerization, and morphologies of the nanoribbons changed significantly, depending on the OEG chain length. Taken together, our findings open a new avenue for the enzymatic reaction-based facile production of novel cellulosic soft materials with regular nanostructures.