Heavily Cu-doped Bi2Te3 nanoparticles were prepared by intercalating copper metal into flower-like Bi2Te3 nanoparticles using the disproportionation redox reaction of Cu(I) salt. The phase, chemical composition, and morphology of the Bi2Te3 nanoflowers were analyzed by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The synthesized Cu-doped Bi2Te3 nanopowders were consolidated by spark-plasma sintering into bulk pellets, and the effects of Cu-doping on the transport properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of these materials were investigated. Superstoichiometric amounts of Cu (up to ∼28 at%) can be incorporated into flower-like Bi2Te3 nanoparticles, which have large accessible surface area for diffusion of Cu ions. The flower-like morphologies did not change despite high Cu incorporation. Variation in carrier concentration was achieved by changing Cu precursor concentration. Cu-doping in Bi2Te3 can enhance the Seebeck coefficient due to a decrease in carrier concentration, thus the power factors increased compared with that of the un-doped sample. Furthermore, the thermal conductivity of Cu-doped Bi2Te3 is substantially reduced. As a result, Cu-doped Bi2Te3 sample with 15.6 at% Cu exhibited the best thermoelectric performance with a figure of merit of 0.67 at 415 K, which is more than two times higher than that of undoped Bi2Te3 nanopowder.