The　integration　of　photoacoustic　imaging　(PAI)　and　photothermal　therapy　(PTT)　within　the　second　near-infrared　(NIR-II)　window,　offering　a　combination　of　high-resolution　imaging　and　precise　non-invasive　thermal　ablation,　presents　an　attractive　opportunity　for　cancer　treatment.　Despite　the　significant　promise,　the　development　of　this　noninvasive　phototheranostic　nanomedicines　encounters　challenges　that　stem　from　tumor　thermotolerance　and　limited　therapeutic　efficacy.　In　this　contribution,　we　designed　an　amphiphilic　semiconducting　polymer　brush　(SPB)　featuring　a　thermosensitive　carbon　monoxide　(CO)　donor　(TDF-CO)　for　NIR-II　PAI-assisted　gas-augmented　deep-tissue　tumor　PTT.　TDF-CO　nanoparticles　(NPs)　exhibited　a　powerful　photothermal　conversion　efficiency　(43.1%)　and　the　capacity　to　trigger　CO　release　after　NIR-II　photoirradiation.　Notably,　the　liberated　CO　not　only　acts　on　mitochondria,　leading　to　mitochondrial　dysfunction　and　promoting　cellular　apoptosis　but　also　hinders　the　overexpression　of　heat　shock　proteins　(HSPs),　enhancing　the　tumor’s　thermosensitivity　to　PTT.　This　dual　action　accelerates　cellular　thermal　ablation,　achieving　a　gas-augmented　synergistic　therapeutic　effect　in　cancer　treatment.　Intravenous　administration　of　TDF-CO　NPs　in　4T1　tumor-bearing　mice　demonstrated　bright　PAI　signals　and　remarkable　tumor　ablation　under　1064 nm　laser　irradiation,　underscoring　the　potential　of　CO-mediated　photothermal/gas　synergistic　therapy.　We　envision　this　tailor-made　multifunctional　NIR-II　light-triggered　SPB　provides　a　feasible　approach　to　amplify　the　performance　of　PTT　for　advancing　future　cancer　phototheranostics.　Graphical　abstract:　(Figure　presented.)　©　The　Author(s)　2024.