Protein polymers of covalently cross-linked protein monomers are highly attractive biomaterials because each monomer unit possesses distinct protein functions. Protein polymers often show enhancement effects on the function by integrating a large number of molecules into one macromolecule. The cross-linking site of component proteins should be precisely controlled to avoid diminishing the protein function. However, preparing protein polymers that are cross-linked site-specifically with a high cross-linking degree is a challenge. Here, we demonstrate the preparation of a site-specifically cross-linked protein polymer that has a hyperbranched polymer-like structure with a high cross-linking degree. A horseradish peroxidase (HRP) reaction was used to achieve the protein polymerization through a peptide tag containing a tyrosine residue (Y-tag). Y-tag sequences were introduced to both N- and C-termini of a model protein, protein G. The dual Y-tagged protein G (Y-pG-Y) was treated with HRP to form a Y-pG-Y polymer possessing average and maximum cross-linking degree of approximately 70-mer and 150-mer, respectively. The Y-pG-Y polymer shows the highest cross-linking degree among the protein polymers reported, which are completely soluble in water and cross-linked via covalent bonding. The Y-pG-Y was cross-linked site-specifically at the Tyr residue in the Y-tag, retaining its function, and the Y-pG-Y polymer showed extremely strong avidity against immunoglobulin G. The reactivities of N- and C-terminal Y-tags were evaluated, and we revealed that the difference in the radical formation rate by HRP was the key for yielding highly cross-linked protein polymers.