The two-dimensional metal forming at the interface between an oxide insulatorand SrTiO3 provides new opportunities for oxide electronics. However, the quantum Hall effect, one of the most fascinating effects of electrons confined in two dimensions, remains underexplored at these complex oxide heterointerfaces. Here, we report the experimental observation of quantized Hall resistance in a SrTiO3 heterointerface based on the modulation-doped amorphous-LaAlO3/SrTiO3 heterostructure, which exhibits both highelectron mobility exceeding 10, 000 cm2/V s and low carrier density on the order of ~1012 cm-2. Along with unambiguous Shubnikov-de Haasoscillations, the spacing of the quantized Hall resistance suggests that the interface is comprised of a single quantum well with ten parallel conducting two-dimensional subbands. This provides new insight into the electronic structure of conducting oxide interfaces and represents an important step towards designing and understanding advanced oxide devices.