A side‐chain type benzoxazine‐functional cellulose has been developed using click chemistry via the reaction of ethynyl‐monofunctional benzoxazine monomer and azide‐functional cellulose. The synthesis, crosslinking, and thermal properties of the benzoxazine‐functional cellulose are studied by NMR, FTIR, DSC, and TGA. The crosslinking reaction of the benzoxazine side‐chain unusually takes place at low‐temperatures in comparison to an ordinary benzoxazine resins. Upon crosslinking, the polymer shows high char yield of 40%, which is a marked improvement from a mere 4% of the unfunctionalized cellulose. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012