On-chip　optical　switches　have　emerged　as　a　new　class　of　photonic　components　for　high-performance　optical　communication　networks　and　on-chip　interconnects,　in　which　the　all-optical　configuration　without　the　incorporation　of　other　control-means　is　highly　desired.　While　two-dimensional　(2D)　ultrathin　materials　demonstrate　their　great　potential　in　developing　ultrafast　all-optical　switches　owing　to　their　unique　light-matter　interaction,　such　investigations　have　so　far　been　limited　to　the　fiber　optic　platform　or　free　space.　Here,　we　realize　an　all-optical　on-chip　switch　from　a　silicon　waveguide-based　asymmetric　Mach-Zehnder　interferometer　(MZI)　structure　enabled　by　2D　ultrathin　Ga2S3.　Upon　the　visible　light　excitation　at　532　nm,　excessive　photocarriers　in　Ga2S3　cause　a　change　of　the　refractive　index　and　subsequently　a　phase　variation　between　MZI　arms　at　the　1550　nm　operation　wavelength,　triggering　on　the　optical　switch.　On　the　other　hand,　the　switch　is　off　without　the　visible　light　stimulation,　as　the　phase　variation　is　recovered　due　to　the　ultrafast　photo-exciton　relaxation　behavior　of　Ga2S3.　The　Ga2S3-enabled　all-optical　switch　is　driven　at　an　extremely　small　optical　power　density　of　0.12　W　cm(-2)　and　exhibits　a　response　and　recovery　time　of　26.3　and　43.5　mu　s,　respectively,　which　as　a　combination　is　superior　to　those　of　fiber　optic-based　all-optical　switches　enabled　by　2D　materials.　This　work　may　provide　a　viable　approach　to　develop　on-chip　all-optical　photonic　components　for　practical　integrated　photonic　chips.