We report that atomically thin two-dimensional silicon quantum sheets(2D Si QSs),prepared by a scal-able approach coupling chemical delithiation and cryo-assisted exfoliation,can serve as a high-performance brain photonic nanoagent for orthotopic glioma theranostics.With the lateral size of approximately 14.0 nm and thickness of about 1.6 nm,tiny Si QSs possess high mass extinction coeffi-cient of 27.5 L g-1 cm-1 and photothermal conversion efficiency of 47.2％at 808 nm,respectively,concur-rently contributing to the best photothermal performance among the reported 2D mono-elemental materials(Xenes).More importantly,Si QSs with low toxicity maintain the trade-off between stability and degradability,paving the way for practical clinical translation in consideration of both storage and action of nanoagents.In vitro Transwell filter experiment reveals that Si QSs could effectively go across the bEnd.3 cells monolayer.Upon the intravenous injection of Si QSs,orthotopic brain tumors are effec-tively inhibited under the precise guidance of photoacoustic imaging,and the survival lifetime of brain tumor-bearing mice is increased by two fold.Atomically thin Si QSs with strong light-harvesting capabil-ity are expected to provide an effective and robust 2D photonic nanoplatform for the management of brain diseases.