Formation　of　protein-polymer　conjugates　(PPCs)　is　critical　for　many　studies　in　chemical　biology,　biomedicine,　and　enzymatic　catalysis.　Polymers　with　coordinated　physicochemical　properties　confer　synergistic　functions　to　PPCs　that　overcome　the　inherent　limitation　of　proteins.　However,　application　of　PPCs　has　been　synthetically　restricted　by　the　limited　modification　sites　and　polymer　grafting　method.　Here,　we　present　a　versatile　strategy　for　site-selective　PPC　synthesis.　The　initiator　was　specifically　tethered　to　the　preoxidized　glycan　moieties　through　oxime　chemistry.　Polymer　brushes　were　grown　in　situ　from　the　glycan　by　atom-transfer　radical　polymerization　to　generate　well-controlled　PPCs.　Notably,　the　modification　is　site-specific,　multivalent,　and　alterable　depending　on　protein　glycosylation.　Additionally,　we　demonstrated　that　the　cytocompatible　method　enabled　the　growth　of　polymer　chains　from　the　surface　of　living　yeast　cells.　These　results　verified　a　facile　technology　for　surface　modification　of　biomacromolecules　by　desired　polymers　for　various　biomedical　applications.　©　2021　American　Chemical　Society.