In　recent　years,　rare-earth-based　chalcogenides　have　gained　attention　promising　materials　in　the　field　of　infrared　nonlinear　optical　(IR-NLO)　applications　owing　to　their　exceptional　physicochemical　properties.　However,　they　frequently　encounter　challenges　such　as　adverse　two-photon　absorption　and　low　laser-induced　damage　thresholds　(LIDTs)　caused　by　narrow　optical　band　gaps　(Eg),　which　limit　their　practical　utility.　In　this　study,　we　started　with　the　centrosymmetric　(CS)　parent　compound　EuGa2S4　to　develop　two　new　noncentrosymmetric　(NCS)　Eu-based　chalcogenides,　namely,　EuZnSiS4　and　EuCdSiS4,　employing　a　rational　cross-substitution　strategy.　Despite　having　identical　stoichiometry,　both　compounds　crystallize　in　distinct　NCS　orthorhombic　space　groups　[Fdd2　(no.　43)　vs　Ama2　(no.　40)],　as　confirmed　by　single-crystal　structure　analysis.　Their　crystal　structures　feature　highly　distorted　tetrahedral　motifs　interconnected　via　corner-sharing,　forming　unique　two-dimensional　layers　that　host　Eu2+　cations.　Furthermore,　both　compounds　exhibit　robust　phase-matching　second-harmonic　generation　(SHG)　intensities　of　1.5　×　AgGaS2　for　EuZnSiS4　and　2.8　×　AgGaS2　for　EuCdSiS4　under　2050　nm　excitation.　They　also　demonstrate　high　LIDTs　(approximately　14-17　×　AgGaS2),　wide　Eg　(＞2.5　eV),　and　transparency　windows　extending　up　to　18.2　μm.　Particularly　noteworthy,　EuCdSiS4　stands　out　as　a　pioneering　example　in　the　Eu-based　IR-NLO　system　for　successfully　combining　a　broad　Eg　(＞2.56　eV,　equivalent　to　that　of　AgGaS2)　with　a　significant　SHG　effect　(＞1.0　×　AgGaS2)　simultaneously.　Structural　analyses　and　theoretical　insights　underscore　that　the　reasonable　combination　of　asymmetric　functional　units　plays　a　pivotal　role　in　driving　the　CS-to-NCS　structural　transformation　and　enhancing　the　NLO　and　linear　optical　properties　of　these　Eu-based　chalcogenides.　This　study　presents　a　promising　chemical　pathway　for　advancing　rare-earth-based　functional　materials　and　suggests　exciting　opportunities　for　their　future　applications　in　IR-NLO　technologies.　©　2024　American　Chemical　Society.