The　atomically　thin　group　III　monochalcogenides　have　emerged　as　potential　candidates　for　nanoscale　optoelectronic　applications　due　to　their　tunable　bandgaps　and　high　carrier　mobility.　In　this　work,　by　means　of　ab　initio　calculations,　we　have　systematically　investigated　the　geometrical　structures,　electronic　structures,　and　optical　properties　of　the　orthogonal　phase　(O-phase)　group　III　monochalcogenides　MX　(M　=　Ga　and　In,　X　=　S,　Se　and　Te)　monolayers,　nanoribbons,　heterostructures　and　their　potential　applications　as　photocatalysts　for　water　splitting.　It　is　highlighted　that　the　two-dimensional　(2D)　O-phase　MX　monolayers　not　only　are　dynamically　and　thermally　stable,　but　also　exhibit　distinguished　optical　properties　(∼105　cm-1)　with　broad　absorption　spectrum　in　the　visible　and　ultraviolet　light　regions.　Furthermore,　it　is　noted　that　nano-structure　designing　can　further　modulate　their　electronic　structures　in　desirable　ways.　For　instance,　the　bandgap　of　O-phase　GaTe　is　relevant　to　the　width　of　1D　nanoribbon.　On　the　other　hand,　the　type-II　InSe/GaTe　and　InTe/GaTe　heterostructures　confine　the　photo-generated　electrons　and　holes　at　the　opposite　parts,　which　is　beneficial　for　the　separation　of　hydrogen　and　oxygen　during　the　photocatalysis　water　splitting　process.　©　2019　IOP　Publishing　Ltd.