Polymeric　carbon　nitrides　(PCNs),　usually　the　melon　phase,　have　been　extensively　applied　as　photocatalysts　for　CO2　reduction;　however,　their　performance　is　still　unsatisfactory.　The　condensed　allotrope,　namely,　poly(triazine　imide)　(PTI)　with　extended　conjugation　and　a　crystallized　structure,　indeed　holds　more　favorable　compositional　and　structural　advantages　for　photocatalytic　CO2　reduction　but　remains　to　be　fully　exploited.　Herein,　hexagonal　prism-shaped　PTI　crystals　were　synthesized　and　developed　as　a　high-performance　photocatalyst　for　CO2　reduction.　With　Co(bpy)32+　as　a　cocatalyst,　the　PTI　crystals　exhibit　a　CO　evolution　rate　of　44　μmol　h-1　(i.e.,　1467　μmol　g-1　h-1)　with　93%　selectivity,　markedly　superior　to　that　of　the　melon　counterpart.　Moreover,　PTI　crystals　manifest　an　apparent　quantum　efficiency　of　12.9%　at　365　nm,　representing　the　state-of-the-art　value　by　PCN　photocatalysts　for　CO2-to-CO　reduction　without　using　noble　metals.　The　surface　pyridine　N　species　of　PTI　are　exposed　as　active　sites　to　dominate　CO2　activation　and　conversion,　which,　together　with　the　high　crystallinity　to　facilitate　charge　separation　and　transport,　endows　high　CO2　reduction　efficiency.　In　situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy　determines　the　key　intermediates　during　the　CO2　reduction　reaction　and,　consequently,　constructs　the　possible　reaction　mechanism.　©　2025　American　Chemical　Society.