Infrared　nonlinear　optical　(IR-NLO)　crystals　possessing　excellent　comprehensive　performance　are　highly　desirable,　yet　their　preparation　remains　extremely　challenging.　Particularly,　inorganic　chalcogenides　with　diamond-like　(DL)　structures　provide　a　tunable　material　platform　for　their　structural　design　and　functional　control.　In　this　work,　a　strategy　involving　the　construction　of　chalcogenides　with　DL　structures　using　the　strong　polarizability　of　metal　cations　has　been　put　forward;　thus,　a　quaternary　Hg-containing　metal　sulfide　HgCuPS4　has　been　successfully　discovered　by　the　high-temperature　solid-state　technology.　A　remarkable　structural　characteristic　of　HgCuPS4　is　the　three-dimensional　(3D)　defect　DL　framework　constructed　by　vertex-sharing　alignments　of　asymmetric　building　motifs　(ABMs).　The　combination　of　the　unique　defect　DL　structure　and　the　strong　polarizability　of　the　Hg2+　cations　enables　such　compound　to　achieve　phase　matchability　in　the　IR　range　with　a　high　laser-induced　damage　threshold　(4.2　×　AgGaS2)　and　a　strong　second　harmonic　generation　response　(dij　=　6.5　×　AgGaS2),　the　best　among　the　quaternary　DL　chalcogenides　reported　so　far.　Moreover,　the　detailed　local　dipole　moment　calculations　and　the　theoretical　results　based　on　the　length-gauge　formalism　elucidate　that　the　very　high　dij　value　of　HgCuPS4　originates　from　the　combined　effects　of　distorted　[HgS4],　[CuS4],　and　[PS4]　ABMs,　that　is,　the　3D　defect　DL　structure.　This　discovery　can　effectively　help　understand　and　design　other　promising　defect　DL　metal　chalcogenides　toward　future　high-performing　IR-NLO　applications.　©　2020　American　Chemical　Society.