Nanoporous silica membranes exhibit excellent H2/CO2separation properties for sustainable H2production and CO2capture but are prepared via complicated thermal processes above 400 °C, which prevent their scalable production at a low cost. Here, we demonstrate the rapid fabrication (within 2 min) of ultrathin silica-like membranes (∼3 nm) via an oxygen plasma treatment of polydimethylsiloxane-based thin-film composite membranes at 20 °C. The resulting organosilica membranes unexpectedly exhibit H2permeance of 280–930 GPU (1 GPU = 3.347 × 10–10mol m–2s–1Pa–1) and H2/CO2selectivity of 93–32 at 200 °C, far surpassing state-of-the-art membranes and Robeson’s upper bound for H2/CO2separation. When challenged with a 3 d simulated syngas test containing water vapor at 200 °C and a 340 d stability test, the membrane shows durable separation performance and excellent hydrothermal stability. The robust H2/CO2separation properties coupled with excellent scalability demonstrate the great potential of these organosilica membranes for economic H2production with minimal carbon emissions.