This work presents sodium poly(heptazine imide) (NaPHI)‐based materials, synthesized in a NaCl medium, as highly effective platforms for CO 2 capture. High crystallinity—an often‐overlooked aspect in PHI frameworks—is identified as a key factor governing CO 2 adsorption capacity in microporous structures. Thermogravimetric analysis and manometric studies reveal a CO 2 uptake of ≈3.8 mmol g −1 , at 1 bar and 25 °C, surpassing most reported PHI‐based adsorbents under similar conditions. Exchanging Na + with K + or Rb + preserves CO 2 adsorption performance, whereas Cs + incorporation induces structural distortion, greatly reducing CO 2 adsorption capacity in PHI. These materials exhibit excellent cyclic stability (20 cycles) without degradation and CO 2 adsorption capacity loss. Notably, at flue gas‐relevant temperature (100 °C), NaPHI attains a CO 2 capacity of 2.1 mmol g −1 , doubling the performance of benchmark Zeolite 13X (1.1 mmol g −1 ). Ideal Adsorbed Solution Theory confirms remarkable CO 2 /N 2 selectivity (≈3.8 mmol g −1 vs typical N 2 adsorption of 0.3 mmol g −1 ), a critical property for postcombustion CO 2 capture. These findings position PHI‐based materials as a disruptive platform for CO 2 adsorption, offering 1) straightforward synthesis from readily available precursors, 2) promising scalability, and 3) outstanding performance.