A novel mixed-valence Cu(I)-Cu(II) coordination polymer with an infinite two-dimensional structure, [Cu3(I)Cu(II)Br3(3,5-Dmpip-dtc)2]n (3,5-Dmpip-dtc(-) = 3,5-dimethylpiperidine dithiocarbamate), was prepared, and its structure was characterized by X-ray diffraction. It shows semiconducting behavior with a relatively small activation energy and high carrier mobility, which were investigated using impedance spectroscopy and flash photolysis time-resolved microwave conductivity measurements, respectively.