A highly versatile seed-mediated approach for the synthesis of citrate-stabilized gold, silver and palladium nanoparticles (NPs) with size control is reported. The use of iron(II) as a reducing agent enables the fabrication of monodisperse NPs in a wide range of sizes (from 15 nm to at least 120 nm (90 nm for Pd)) at room temperature. The citrate as capping ligand on the NPs surface facilitates its further surface modification with proteins and thiolated molecules.