We report a novel electrochemical approach for synthesizing colloidal silver in an aqueous phase by a hemoglobin-modified boron-doped diamond electrode. The resulting Ag nanoparticles are within 10 nm in size and highly monodisperse with minimal electrode deposition. We also introduce a method for measuring the yield of synthesized nanoparticles using square-wave voltammetry as an alternative to UV-vis spectroscopy. More than 50% of the transferred electrons contributed directly to the formation of silver nanoparticles. This high yield indicates that such electrochemical synthesis is an efficient one-pot method for producing colloidal silver free of toxic reagents and offers a path toward green metal nanoparticle synthesis in solution. A comparative study using alternative electrodes, modifiers, and surfactants suggests a mechanism for the formation of silver nanoparticles mediated by hemoglobin-modified boron-doped diamond electrodes.