Potassium poly(heptazine imide) (PHI) is a photocatalytically active carbon nitride material that was recently prepared from substituted 1,2,4‐triazoles. Here, we show that the more acidic precursors, such as commercially available 5‐aminotetrazole, upon pyrolysis in LiCl/KCl salt melt yield PHI with the greatly improved structural order and thermodynamic stability. Tetrazole‐derived PHIs feature long‐range crystallinities and unconventionally small layer stacking distances, leading to the altered electronic band structures as shown by Mott–Schottky analyses. Under the optimized synthesis conditions, visible‐light driven hydrogen evolution rates reach twice the rate provided by the previous gold standard, mesoporous graphitic carbon nitride, which has a much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared with ordinary carbon nitrides makes it an efficient water oxidation photocatalyst, which works even in the absence of any metal‐based co‐catalysts under visible light. To our knowledge, this is the first case of metal‐free oxygen liberation from water. Highly crystalline potassium poly(heptazine imides) are easily prepared by the pyrolysis of amino‐tetrazole in eutectic LiCl/KCl salt mixtures. Owing to the suitable positions of their conduction and valence bands, the materials are efficient photocatalysts for Pt‐assisted hydrogen and co‐catalyst free oxygen evolution from water under visible‐light irradiation.