Tannic acid (TA) based materials have received significant interest owing to their broad spectrum of chemical and biological properties. Herein, a novel tannic acid based hydrogel, TA–PEG hydrogel, was synthesized via Mitsunobu polymerization/polycondensation, in which TA and polyethylene glycol (PEG) were simply crosslinked together by ether linkages. This method was performed in one pot, straightforward, metal free and robust, ignoring the strong ionic/hydrophobic interactions between tannic acid and PEG. Bearing catechol and pyrogallol units from TA, TA–PEG hydrogel did not only reduce the silver and gold precursor, but also served as a capping agent and stabilizer for the in situ formed Au and Ag nanoparticles (NPs). Furthermore, the antioxidant activity of the hydrogel was excellent (94%) in the case of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging. TA–PEG hydrogel also showed antibacterial activity against Staphylococcus aureus and Escherichia coli. This work suggested a new method leading to polyphenol based soft materials rather than a complex coacervated microstructure. The resulting TA–PEG hydrogel has potential application in biomedical materials.