The　knowledge　concerning　the　influence　of　defects　on　the　nonlinear　optical　response　of　materials　remains　scarce　so　far.　In　this　work,　we　have　successfully　introduced　defects　into　SnS2　nanosheets　by　plasma　treatment　and　shown　that　a　defect　generation　is　an　effective　approach　to　significantly　improve　the　reverse　saturable　absorption　of　SnS2.　The　SnS2　nanosheets　treated　with　Ar　plasma　for　40　s　exhibit　a　nonlinear　absorption　coefficient　(β0)　as　large　as　(2.9　±　0.12)　×　104　cm　GW-1,　which　is　nearly　9　times　that　of　the　untreated　sample.　The　influence　of　Ar-plasma-treatment　time,　defect　type,　and　defect　number　on　the　nonlinear　absorption　of　SnS2　nanosheets　are　also　studied.　Structure　and　spectroscopy　characterization　confirms　the　introduction　of　S　and　Sn　vacancies　with　Ar-plasma　etching.　Surface　photovoltage　spectroscopy　and　density　functional　theory　calculation　indicate　that　S　vacancies　can　induce　in-gap　states　in　the　band　gap.　These　in-gap　states　act　as　intermediate　states　for　the　successive　absorption　of　photons　during　femtosecond　laser　excitation　(namely,　excited-state　absorption).　In　contrast,　Sn　defects　cannot　lead　to　in-gap　states　and　have　a　limited　contribution　to　nonlinear　absorption.　Our　result　would　provide　a　promising　way　to　improve　optical　nonlinearities.　©