Herein　it　is　reported　that　electrochemical　ion-intercalation　is　a　convenient　and　effective　strategy　toward　materials　with　giant　nonlinear　optical　(NLO)　absorption.　Alkali-metal　ions　(i.e.,　Li+,　Na+,　K+)　are　electrochemically　intercalated　into　SnS2　nanosheets.　All　ion-intercalated　samples　exhibit　remarkably　enhanced　optical　nonlinearity　compared　with　an　untreated　sample,　and　Li-intercalated　SnS2　(Li0.952SnII0.398SnIV0.563S2)　possesses　optimized　strong　NLO　performance.　Li0.952SnII0.398SnIV0.563S2　exhibits　strong　saturable　absorption,　and　the　corresponding　nonlinear　absorption　coefficient　(βeff)　is　-1.7　×　104 cm　GW–1　for　the　laser　excitation　at　515 nm.　Li0.952SnII0.398SnIV0.563S2　shows　prominent　reverse　saturable　absorption　with　the　laser　excitation　at　800 nm　(βeff:　2.8　×　104 cm　GW–1)　and　1030 nm　(βeff:　1.4　×　104 cm　GW–1).　All　βeff　values　are　larger　than　most　of　the　reported　inorganic　NLO　materials　at　corresponding　wavelengths.　The　optical　limiting　threshold　of　Li0.952SnII0.398SnIV0.563S2　is　8　×　10–4　J　cm–2,　two　orders　of　magnitude　smaller　(better)　than　the　bench-mark　composite　(e.g.,　SWNT-NH-TPP).　Ion　intercalation　introduces　abundant　in-gap　defects.　The　excitation　of　electrons　in　in-gap　states　to　conduction　band　intensifies　the　Pauli-blocking　effect　and　therefore　promotes　the　saturable　absorption　under　the　515 nm　laser　excitation,　while　the　in-gap　defect　states　acting　as　effective　excitation　pathway　facilitate　excited-state　absorption　for　800　and　1030　nm　laser.　©　2021　Wiley-VCH　GmbH