A novel hydrophilic phosphine ligand for a ruthenium catalyst was synthesized towards useful living radical polymerization in water. Triethylene glycol (TEG) was introduced at the para position of triphenylphosphine and the resultant TEG-decorated ligand was combined with a Cp*-based ruthenium precursor ([Cp*Ru(μ₃-Cl)]₄) to prepare a hydrophilic ruthenium complex. The obtained ruthenium complex induced homogeneous controlled polymerization of poly(ethylene glycol) methacrylate (PEGMA) in water: the monomer conversion reached over 90% in 20 min, and the obtained polymers showed narrow MWD (M_w/M_n < 1.16). The ruthenium catalyst was active enough to allow a decrease in catalyst amount ([Ru]₀/[initiator]₀ = 1/100) or polymerization at low temperature, 0 °C [for 2-hydroxyethyl methacrylate (HEMA)]. The catalyst also induced aqueous block copolymerization for the combination of water-soluble monomers: PEGMA/HEMA and PEGMA/2-(dimethylamino)ethyl methacrylate (MeDMA).