AbstractCrystalline and amorphous organic materials are an emergent class of heterogeneous photocatalysts for the generation of hydrogen from water, but a direct correlation between their structures and the resulting properties has not been achieved so far. To make a meaningful comparison between structurally different, yet chemically similar porous polymers, two porous polymorphs of a triazine‐based graphdiyne (TzG) framework are synthesized by a simple, one‐pot homocoupling polymerization reaction using as catalysts CuI for TzGCu and PdII/CuI for TzGPd/Cu. The polymers form through irreversible coupling reactions and give rise to a crystalline (TzGCu) and an amorphous (TzGPd/Cu) polymorph. Notably, the crystalline and amorphous polymorphs are narrow‐gap semiconductors with permanent surface areas of 660 m2 g−1 and 392 m2 g−1, respectively. Hence, both polymers are ideal heterogeneous photocatalysts for water splitting with some of the highest hydrogen evolution rates reported to date (up to 972 μmol h−1 g−1 with and 276 μmol h−1 g−1 without Pt cocatalyst). Crystalline order is found to improve delocalization, whereas the amorphous polymorph requires a cocatalyst for efficient charge transfer. This will need to be considered in future rational design of polymer catalysts and organic electronics.