Sn-based SBA-15 was prepared by reacting di-n-butyldimethoxystannane with SBA-15 pretreated with trimethylchlorosilane (TMCS) to cap the external hydroxyl groups. Small-angle X-ray diffraction (SXRD), infrared spectroscopy (IR), nitrogen adsorption/desorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma atomic emission (ICP-AES) measurements allow us to propose that the organotin species are located within the pore channels of the mesoporous host. This novel material catalyzes selectively the coupling of CO2 with methanol to dimethyl carbonate (DMC). The reaction time–conversion dependence shows that a turnover number (TON) of 16 can be reached at 423 K under 20 MPa, which is among the highest reported so far in the absence of water traps. Moreover, as the catalytic activity is retained after recycling, even higher values can be obtained on a cumulative basis. A further TON increase is observed with the reaction temperature. Interestingly, the tin-based SBA-15 mesoporous material exhibits lower TONs if the TMCS pretreatment is left out. Therefore, the organotin species located outside the channels are far less active than those located within.