The synthesis of the multifunctional phosphorus-based flame-retardant crosslinking agent is presented here. Gallic acid was selected as a raw material for the production of the desired product because of its availability (bio-based), multifunctional structure, and applicability of the various chemistries. Along with the carboxylic acid group, hydroxyl groups also have the tendency to react with the replaceable halogenated compounds and it was necessary to protect the hydroxyl groups by acetylation. The synthesis procedure follows acetylation, reaction with phenylphosphonic dichloride (PPDC), and deacetylation to obtain the final product (GA-P). The structure confirmation and the progress of the reactions were confirmed using hydroxyl and acid values, Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. The formed product was used as a crosslinking agent to produce the polyurethane coatings with different loadings and various thermal, mechanical, and flame-retardant properties that were studied. The thermal and the flame-retardant properties showed significant increase with increasing concentration of GA-P which were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), limiting oxygen index (LOI), and UL-94 tests. The coating with the highest concentration of GA-P showed 27 LOI and self-extinguishing behavior within 10 s of ignition. The mechanical properties deteriorated with increasing concentration of GA-P due to the increased brittleness and crosslinking density.