The adlayer structures of benzotriazole (BTA) on Cu(110) (100), and (111) were investigated by using in situ scanning tunneling microscopy in aqueous HClO4 solution. BTA molecules were aligned in an orderly manner on well-defined Cu single-crystal surfaces, and the adlayer structure was dependent on the crystallographic orientation of the Cu surface. The BTA molecules were adsorbed in parallel to the surfaces of Cu(110) and (100), whereas on Cu(111) they stacked vertically to form molecular rows. The molecular orientation of BTA was found to influence electrochemical reactions, such as the hydrogen evolution reaction on the Cu electrodes.