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Membranes with precise molecular sieving channels that break the permeability‐selectivity trade‐off are desirable for energy‐efficient gas separation. Two‐dimensional (2D) membranes sieve gas through their special interlayer channels between neighboring nanosheets. However, the regulation and precise control of the nanochannels that match well with the size of the gas molecules remains a big challenge. Herein, accurate tuning of the interlayer spacing of layered double hydroxide (LDH) membranes at sub‐nanometer level was achieved by intercalation of Cl−, Br−, I−, and NO3− ions. Such high‐precision control allows customizable gas separation by selecting specific LDH membranes with appropriate channels according to the size of the gas molecules. Two membranes were used for demonstration: Cl‐LDH membrane shows high H2 permeance of ∼1870 GPU and desirable selectivities for H2/CO2(81), H2/N2(197), H2/CH4(320), and H2/C3H8(603); while I‐LDH membrane displays CO2 permeance of ∼1780 GPU and CO2/N2, CO2/CH4 selectivities of 182 and 297, respectively. The simultaneously high permeabilities and selectivities surpass the 2008 Robeson upper bounds. Molecular dynamics simulations quantitatively support the experiment results, further confirming the significant role of interlayer anions in the regulation of gas‐sieving channels. Given the rich variability of layered spacing and interlayer microenvironment for LDH materials, this work provides a platform membrane for various molecular sieving, including gas separation, solvent purification, seawater desalination, and so on. [ABSTRACT FROM AUTHOR]