The structure of poly-tetraruthenated nickel porphyrin was unveiled for the first time, by electrochemistry, Raman spectroelectrochemistry and hydroxyl radical trapping assay. The electrocatalytic active material, precipitated on the electrode surface after successive cycling of [NiTPyP{Ru(bipy)2Cl}4]4+ species in strong aqueous alkaline solution (pH=13), was found to be a peroxo bridged coordination polymer. The electropolymerization process involves hydroxyl radicals (as confirmed by the characteristic set of DMPO/•OH adduct EPR peaks) as reaction intermediates, electrocatalytically generated in the 0.80-1.10 V range, that induce the formation of Ni-O-O-Ni coordination polymers, as evidenced by Raman spectroelectrochemistry and molecular modeling studies. The film growth is halted above 1.10 V due to the formation of oxygen gas bubbles.