A new three-dimensional microporous metal–organic framework (MOF) Zn(BDC-OH)(DABCO)0.5·(DMF)2(H2O) (UTSA-25; H2BDC-OH = 2-hydroxybenzenedicarboxylic acid, DABCO = 1,4-diazabicyclo[2.2.2]octane) with functional–OH groups on the pore surfaces was solvothermally synthesized and structurally characterized. UTSA-25 features a three-dimensional structure with 3D intercrossed channels of about 7.5 × 7.5, 3.2 × 4.7, and 3.2 × 4.7 Å2, respectively. The small pores and the functional –OH groups on the pore surfaces within the activated UTSA-25a have enabled their strong interactions with CO2 of adsorption enthalpy of 22.5 kJ mol–1, which is higher than that of 17.5 kJ mol–1 in the original MOF Zn(BDC)(DABCO)0.5 without the functionalized –OH groups. Accordingly, CO2/CH4 separation selectivities in UTSA-25a of 17.2 and 12.5 at 273 and 296 K, respectively, are much higher than those of 4.4 and 3.7 in Zn(BDC)(DABCO)0.5, thus highlighting UTSA-25a as a very promising porous material for industrially important CO2/CH4 separation.