One of the bottlenecks in realizing the potential of atom-thick graphene membrane for gas sieving is the difficulty in incorporating nanopores in an otherwise impermeable graphene lattice, with an angstrom precision at a high-enough pore density. We realize this design by developing a synergistic, partially decoupled defect nucleation and pore expansion strategy using O plasma and O treatment. A high density (ca. 2.1 × 10 cm) of H-sieving pores was achieved while limiting the percentage of CH-permeating pores to 13 to 22 parts per million. As a result, a record-high gas mixture separation performance was achieved (H permeance, 1340 to 6045 gas permeation units; H/CH separation factor, 15.6 to 25.1; H/CH separation factor, 38.0 to 57.8). This highly scalable pore etching strategy will accelerate the development of single-layer graphene-based energy-efficient membranes.