This　study　investigates　the　effects　of　thermal　damage　on　the　tensile　behavior　of　granite　during　Brazilian　splitting　tests,　focusing　on　the　negative　Poisson＇s　ratio　(NPR)　effect.　Using　digital　image　correlation　and　acoustic　emission　techniques,　the　research　reveals　that　increasing　thermal　damage　from　25°C　to　800°C　reduces　granite　tensile　strength　by　up　to　85.7%　and　induces　a　brittle-to-ductile　transition.　The　NPR　effect　emerges,　intensifies,　and　subsequently　disappears　as　temperature　increases,　significantly　altering　stress　distribution　and　deformation　patterns.　Local　contraction　zones　created　by　the　NPR　effect　can　lead　to　overestimation　of　tensile　strength　in　thermally　damaged　rock.　Acoustic　emission　monitoring　demonstrates　a　strong　correlation　between　the　NPR　effect　and　microcrack　development.　These　findings　advance　the　understanding　of　rock　mechanical　behavior　under　thermal　damage　and　provide　practical　insights　for　tensile　strength　evaluation　in　high-temperature　geological　settings,　such　as　deep　underground　energy　development　and　nuclear　waste　disposal.　©　2025　John　Wiley　&　Sons　Ltd.