Thermally rearranged (TR) polymers are an important class of microporous polymers with remarkable gas transport performance, particularly suitable for CO2 permeation and separation over large gas molecules. The fabrication of TR polymers into ultrathin membranes is highly desirable for practical application, but it is very challenging. In this work, a 2D scaffold of graphene oxide (GO) nanosheets was formed inside a TR polymer to assist the fabrication of a defect-free and ultrathin (less than 40 nm) selective layer of thermally rearranged polybenzoxazole-co-imide (TR-PBOI) membranes for energy-efficient CO2 separation. The GO scaffold inside the polymer phase not only enabled the formation of the ultrathin selective layer of TR-PBOI, but also provided mechanical robustness. The resulting membrane showed remarkable gas permeance, while maintaining the gas selectivity of the pristine polymer. In particular, it had a CO2 permeance of 1784 GPU and a CO2/CH4 selectivity of 32, whereas the freestanding TR-PBOI membrane only exhibited a CO2 permeance of 3.7 GPU with a CO2/CH4 selectivity of 35. In other words, the rGO-PBOI (TR-PBOI with reduced GO) membrane has 482 times higher CO2 permeance than the TR-PBOI freestanding membrane at a similar CO2/CH4 selectivity.