Three-dimensional graphene (3DGr) is explored as an unconventional support material for supported ionic liquid membranes (SILMs) in gas separations. Herein, molecular dynamics/grand canonical Monte Carlo (MD/GCMC) simulations were performed to investigate the CO2/CH4 separation performance in porous 3DGr and 3DGr-supported IL ([EMIM][TF2N]) membranes as a function of pressure and IL loading in the 3DGr-IL membrane. Bulk ionic liquid (IL) was treated as a control. Calculations of ratios of Henry's law constants in single-gas simulations and CO2/CH4 membrane selectivities in mixed-gas (1:1 molar ratio) simulations as a function of pressure reveal promising gas separation performance for 3DGr-supported IL (3DGr-IL) membranes. For example, a selectivity of about 24 is observed for the 3DGr-IL membrane with 23 wt % IL loading at high pressures (>3 bar), exceeding the separation performance of bulk IL. Given the excellent mechanical properties of 3DGr, it is demonstrated that it may be used as a mechanically robust support for IL membranes with excellent CO2/CH4 separation performance.