A zeolitic imidazolate framework-8 (ZIF-8) layer was introduced as a surface molecular separator to grant the selective gas-sensing ability to thin-film sensor material composed of SnO2 nanosheets. ZIF-8 possesses a molecular sieving ability, which is in the range of approximately 4.0-4.2 angstrom. Compared to a SnO2 nanosheet gas sensor, a ZIF-8/SnO2 nanosheet gas sensor exhibited a decreased sensor signal response when the gas molecular sizes were large, e.g., C3H6 (4.120 angstrom), 1-C4H8 (5.430 angstrom), and C9H18O (12.011 angstrom). However, the ZIF-8/SnO2 nanosheet sensor signal response to smaller-sized gas molecules, such as C2H4 (3.058 angstrom), was slightly increased due to electron transfer at the ZIF-8 and SnO2 interface. Therefore, the control of gas selectivity based on molecular size was realized by introducing ZIF-8 as a surface molecular separator. These results pave the way for selective gas sensing in various types of applications that depend on gas selectivity.