Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin-film devices requires atomic-scale band-structure engineering based on depolarization-field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO3, it is demonstrated that the surface termination of the ferroelectric in contact with a simple-metal electrode critically affects properties of electroresistance. BaTiO3 barrier-layers with TiO2 or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance-switching ratio in the junctions can be remarkably enhanced up to 10(5) % at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.