With a thin insulator sandwiched between two electrodes, the negative differential resistance (NDR) behavior has been frequently reported for its potential device applications. Here we report the experimental observation of a symmetric NDR characteristic in a resistive switching device based on TiO2. We propose a charge storage mechanism for the NDR effect, with oxygen molecular ions working as the active source, in a thin insulating layer. Current–voltage measurements demonstrated a highly reproducible state at about 0.65 eV, and the photoelectron spectroscopy measurements showed that it complies well with the Ti3d band gap state. Our first-principle calculations confirm that charge storage and release arise from trapping and detrapping of oxygen molecular ions at the defect sites. The results and mechanism demonstrated here in a thin layer could be extended to other systems approaching molecular dimensions for device applications.