A new sustainable route to functionalize silica SBA-15 with 3-aminopropyltrimethoxysilane (3APTS) in supercritical CO2 (scCO2) is proposed. The aminosilane/carbamate salt formed (organic salt) by reaction between CO2 and the primary amine group of 3APTS was solubilized in scCO2 by adding 10% mol ethanol as cosolvent. For the first time such a mixture was used to perform the surface functionalization whilst keeping the advantages of performing the reaction in a supercritical fluid at a moderate temperature. Firstly, the phase behaviour of the 3APTS/carbamate + CO2 + ethanol was studied to select the process experimental conditions. Then, the functionalization experiments were performed at temperatures of 313 and 333 K, pressures ranging from 12.5 to 20.0 MPa and concentration values of 3APTS in the modified scCO2 ranging from 0.5 to 3.5 × 10−3 in molar fraction. The amine functionalized materials were characterized by IR, thermogravimetric analysis, elemental analysis and N2 adsorption isotherms. The effect of pressure and temperature on the amount of 3APTS grafted on silica SBA-15 was not significant. However, as the 3APTS concentration in scCO2 modified with ethanol increased, the grafting density of 3APTS increased gradually and the surface area, pore volume and size of silica SBA-15 were also progressively reduced. The functionalization process in scCO2 was compared to that of the conventional method using toluene. Finally, the performance of the materials for CO2 sorption at low and high-pressure was evaluated. The amine functionalized silica SBA-15 exhibited good CO2 adsorption capacity: 0.7–1.5 mmol g−1 at ambient pressure and 8–12 mmol g−1 at 4.0 MPa. These values indicate the great potential of the amine functionalized silica obtained in scCO2 for carbon capture technology.