Dilatometric measurement of the thermal expansion of water in porous silica shows that the expansion coefficient increases systematically as the pore size decreases below about 15 nm. This behavior is quantitatively reproduced by molecular dynamics (MD) simulations based on a new dissociative potential. According to MD, the structure of the water is modified within approximately 6 A of the pore wall, so that it resembles bulk water at a higher pressure. On the basis of this observation, it is possible to account for the measured expansion, as the thermal expansion coefficient of bulk water increases with temperature over the range considered in this study.