In this study, we developed an open-air atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) as a scalable technique to fabricate polymer-supported silica-based membranes for gas separation. Using the open-air AP-PECVD, we achieved a low-temperature deposition of silica-based molecular sieve membranes onto porous polymeric support in a system that can operate under open-air conditions. We investigated the effect of deposition conditions, such as precursor concentration and deposition temperature, on the membrane structure and permeation property. It was revealed that precursor concentration plays a vital role in determining membrane properties. The membranes prepared with low precursor concentration had an inorganic structure with granular morphology, resulting in poor permselective performance. In contrast, deposition at high precursor concentration led to the formation of a continuous layer with a silicone-like structure. By optimizing the precursor concentration, a membrane having a He permeance of 7.25 × 10−8 mol/(m2 s Pa) with a He/SF6 permeance ratio of 171 can be fabricated at room temperature. The deposition at elevated temperature was also effective in improving the permselective performance. By increasing the deposition temperature to 150 °C, the permselective performance of the membrane was improved to a He permeance of 7.7 × 10−8 mol/(m2 s Pa) and He/SF6 of 434. The resultant membranes also exhibited high selectivity for the separation of industrial relevant gas mixtures such as H2/N2 and H2/CH4 with the permeance ratios of 40 and 38, respectively. The present study demonstrates that open-air AP-PECVD is promising as a scalable technique to fabricate polymer-supported silica-based membranes with high productivity.