Abstract 2D materials have attracted much interest over the past decade in nanoelectronics. However, it was believed that the atomically thin layered materials are not able to show memristive effect in vertically stacked structure, until the recent discovery of monolayer transition metal dichalcogenide (TMD) atomristors, overcoming the scaling limit to sub-nanometer. Herein, the nonvolatile resistance switching (NVRS) phenomenon in monolayer hexagonal boron nitride (h-BN), a typical 2D insulator, is reported. The h-BN atomristors are studied using different electrodes and structures, featuring forming-free switching in both unipolar and bipolar operations, with large on/off ratio (up to 107). Moreover, fast switching speed (\textless15 ns) is demonstrated via pulse operation. Compared with monolayer TMDs, the one-atom-thin h-BN sheet reduces the vertical scaling to ≈0.33 nm, representing a record thickness for memory materials. Simulation results based on ab-initio method reveal that substitution of metal ions into h-BN vacancies during electrical switching is a likely mechanism. The existence of NVRS in monolayer h-BN indicates fruitful interactions between defects, metal ions and interfaces, and can advance emerging applications on ultrathin flexible memory, printed electronics, neuromorphic computing, and radio frequency switches.