The fabrication of organized molecular-scale structures is key to realizing high-performance molecular- and conjugated-polymer devices. Here, we demonstrate a unique single-molecular processing technique using electrochemistry, termed 'electrochemical epitaxial polymerization'. This technique is based on step-by-step electropolymerization of the monomer by applying voltage pulses to a monomer-electrolyte solution that also contains iodine. Using this technique, we observe the formation of high-density arrays of single conjugated-polymer wires as long as 75 nm on the surface of a Au(111) electrode. Our findings unveil the mechanism of electropolymerization, showing that the conjugated polymer wires grow from nuclei adsorbed on the iodine-covered Au( 111) surface. The results may also open the door to mass-production of molecular-scale devices based on conjugated polymers.