Single-layered　nanosheets,　as　an　emerging　class　of　new　materials　with　intriguing　properties　for　potential　applications　in　optoelectronics　and　catalysis,　have　been　widely　investigated　in　recent　years.　However,　these　nanosheets　were　only　confined　to　the　layered　materials　with　out-of-plate　weak　van　der　Waals　bonds,　and　for　non-layered　materials,　it　is　almost　impossible　to　achieve　single-layered　nanosheets　through　the　exfoliation　procedure.　Here,　we　fabricated　freestanding　single-layered　nanosheets　of　the　non-layered　material　γ-Ga2O3　with　unprecedented　thickness　(∼1.25　nm)　by　a　facile　hydrothermal　method　without　using　any　kind　of　shape　control　agents.　The　structural　stability　of　single-layered　γ-Ga2O3　nanosheets　originates　from　the　surface　distortion　and　hydroxyl　group　passivation.　The　prepared　single-layered　γ-Ga2O3　nanosheets　can　enhance　the　photocatalytic　efficiency　of　water　splitting　and　durably　photocatalyze　overall　water　splitting　into　hydrogen　and　oxygen.　For　comparison,　the　common　bulk　γ-Ga2O3　samples　were　inactive　for　oxygen　evolution　in　the　same　case.　This　work　opens　the　door　for　extending　atomically　thick　2D　nanosheets　to　non-layered　compounds　and　holds　promise　for　developing　a　series　of　unprecedented　excellent　properties.　©　2017　The　Royal　Society　of　Chemistry.