Epitaxial heterostructures of lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) exhibit extraordinary quasi-two-dimensional magnetotransport properties at low temperatures. To elucidate the mechanisms responsible for the unique properties of these interfaces, which can guide the design of novel structures having high performances, extensive analyses of the magnetotransport properties at low temperatures are required. We report the magnetotransport properties of a LaAlO3/SrTiO3 system tailored with a topmost surface atomic sheet of titanium dioxide (TiO2). Three unit cells of LaAlO3 were deposited on a reconstructed SrTiO3(001)-(13 × 13)-R33.7° substrate, yielding LaAlO3 covered with a single-atom-thick TiO2 sheet. The high-mobility electrons confined at the LaAlO3/SrTiO3 interface provided significantly high magnetoresistance ratios of +150% and -80% under magnetic fields perpendicular and parallel to the interface, respectively. The in-plane anisotropic magnetoresistance at 4.2 K reached approximately +30%, reflecting the Rashba spin-orbit interactions of the quasi-two-dimensional electrons. A high carrier concentration at the interface realized by the capping of LaAlO3/SrTiO3 with the surface TiO2 sheet significantly contributed to the enhancement of magnetotransport properties arising from the Ti 3d orbitals.