Realization of ideal molecular sieves, in which the larger gas molecules are completely blocked without sacrificing high adsorption capacities of the preferred smaller gas molecules, can significantly reduce energy costs for gas separation and purification and thus facilitate a possible technological transformation from the traditional energy‐intensive cryogenic distillation to the energy‐efficient, adsorbent‐based separation and purification in the future. Although extensive research endeavors are pursued to target ideal molecular sieves among diverse porous materials, over the past several decades, ideal molecular sieves for the separation and purification of light hydrocarbons are rarely realized. Herein, an ideal porous material, SIFSIX‐14‐Cu‐i (also termed as UTSA‐200), is reported with ultrafine tuning of pore size (3.4 Å) to effectively block ethylene (C2H4) molecules but to take up a record‐high amount of acetylene (C2H2, 58 cm3 cm−3 under 0.01 bar and 298 K). The material therefore sets up new benchmarks for both the adsorption capacity and selectivity, and thus provides a record purification capacity for the removal of trace C2H2 from C2H4 with 1.18 mmol g−1 C2H2 uptake capacity from a 1/99 C2H2/C2H4 mixture to produce 99.9999% pure C2H4 (much higher than the acceptable purity of 99.996% for polymer‐grade C2H4), as demonstrated by experimental breakthrough curves. An ideal molecular sieve is realized for the highly efficient removal of C2H2 from C2H2/C2H4 (1:99) mixture, attributed to the optimized pore sizes to efficiently block C2H4 molecules, and strong binding sites to take up a record‐high amount of C2H2, thus simultaneously producing high purity C2H4 (99.9999%) with record C2H4 productivity of 87.5 mmol g−1 per cycle and recovery of C2H2 (97%).