To achieve homogeneous ternary compounds, the growth rates of the component species in a solution must be equal and the processes must be simultaneous. The conditions necessary for the growth of one species should not prevent the growth of the others. Herein we present a synthesis of Cu3Te2Se2 semiconductor nanofilms on indium tin oxide (ITO)-coated polyethylene terephthalate (ITO-PET), ITO-coated glass, and Au plate substrates by an electrochemical underpotential co-deposition (UPCD) method from the same solution at a constant potential. The chemical, morphological, and optical properties of the synthesized nanofilms were determined by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, ultraviolet-visible absorption spectroscopy (UV/Vis), and current-voltage (I-V) studies. The chemical formula of the deposited nanofilms was confirmed as Cu3Te2Se2 by X-ray photoelectron spectroscopy. The SEM images exhibited uniform nano-scale (similar to 40 nm) distribution of the Cu3Te2Se2 nanofilms. The band gaps of the deposited films were determined according to the different deposition times and potentials. The XRD results showed crystalline and single-phase forms of Cu3Te2Se2 nanofilm. Finally, the I-V curves of the Cu3Te2Se2/ITO heterojunctions at room temperature (RT) under normal light conditions indicated matched those of a diode model.