Abstract: Traditional hydrometallurgical methods for recovering spent lithium-ion batteries (LIBs) involve acid leaching to simultaneously extract all valuable metals into the leachate. These methods usually are followed by a series of separation steps such as precipitation, extraction, and stripping to separate the individual valuable metals. In this study, we present a process for selectively leaching lithium through the synergistic effect of sulfuric and oxalic acids. Under optimal leaching conditions (leaching time of 1.5 h, leaching temperature of 70°C, liquid–solid ratio of 4 mL/g, oxalic acid ratio of 1.3, and sulfuric acid ratio of 1.3), the lithium leaching efficiency reached 89.6%, and the leaching efficiencies of Ni, Co, and Mn were 12.8%, 6.5%, and 21.7%. X-ray diffraction (XRD) and inductively coupled plasma optical emission spectrometer (ICP-OES) analyses showed that most of the Ni, Co, and Mn in the raw material remained as solid residue oxides and oxalates. This study offers a new approach to enriching the relevant theory for selectively recovering lithium from spent LIBs.