An organic–inorganic hybrid polymer, composed of poly(methylsilsesquioxane) (PMSSQ) and poly(4‐vinyl benzaldehyde) (PStCHO), is prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization of 4‐vinyl benzaldehyde (StCHO) using a macro‐chain transfer agent (CTA) based on PMSSQ. The obtained PMSSQ–PStCHO is spin‐coated on substrates such as silicon wafers or copper plates to afford aldehyde‐functionalized surfaces. Successful Kabachnik‐Fields post‐polymerization modification (KF‐PMR) of the aldehyde‐functionalized surfaces is conducted with amines and dialkyl phosphonates, and characterized by surface analysis techniques including IR, energy‐dispersive X‐ray (EDX), and X‐ray photoelectron spectroscopy (XPS) measurements, documenting the installation of α‐amino phosphonates onto surfaces with practically quantitative conversion of aldehydes. In addition, the generated α‐amino phosphonates are successfully deprotected to afford the corresponding α‐amino phosphonic acids on surfaces, which make this route a reliable tool‐enabling surface functionalization with α‐amino phosphonic acids.