Two-dimensional borophene nanosheets (2D BNSs) have been theoretically predicted to be an ideal lithium-storage material owing to their high theoretical capacity and low Li-ion diffusion energy barrier. However, the lithium-storage capability and mechanism of 2D BNSs have not yet been reported until now. Herein, ultrathin 2D BNSs (<2 nm) were successfully fabricated by sonication-assisted liquid exfoliation, and further explored as a Li-storage host. The ultrathin BNSs delivered an ultrahigh capacity of 782.9 mAh/g over 500 cycles at 167 mA/g. The Li-storage mechanism of BNSs was revealed by in-situ X-ray diffraction and the results showed that the Li⁺ ions intercalate/deintercalate along (2 0 5) plane with the formation of interstitial compound of Li_0.48B. The charge storage behavior of 2D BNSs was also proved to be dominated by a pseudocapacitance-control process, which contributes to the high-rate capability. This work would accelerate the exploration of emerging 2D materials for high-energy–density lithium-ion batteries.