Amine-functionalized sorbents are promising materials for direct air capture (DAC). Reduced graphene oxide (rGO) aerogels have higher electrical and heat conductivities than commonly studied aminated silica. These properties could jointly enhance the productivity of an associated CO2 separation process. In this study, three-dimensional (3D) rGO aerogels were fabricated, modified with polyethyleneimine (PEI) at various mass ratios, and applied successfully as a CO2 adsorbent at different temperatures and pressures. The structure, morphology, and chemical properties of the aminated composite aerogels based on PEI-rGO were comparably characterized using elemental analysis, thermogravimetric analysis, and scanning electron microscopy. The CO2 uptake performance of the PEI-rGO aerogels was assessed by analysis of the adsorption isotherms at 20 and 50 °C. The optimized PEI-rGO-71 (referring to 71 wt% PEI to the sample) exhibited the highest CO2 uptake across the tested CO2 pressure range (0.03–101 kPa), reaching 0.61 mmol CO2/g at the partial pressure of CO2 in ambient air (0.04 kPa). It showed a superior CO2 capture capacity compared to previously reported amine-modified graphene-based sorbents at ambient CO2 concentration