The influence of nanosilica (nanoscale-size particles of amorphous silica) on the leaching behaviour of Portland cement pastes made with a relatively low water/binder ratio (w/b = 0.36) and different nanosilica contents (0-3.8 wt.% of binder) was investigated through the use of an ultra-accelerated leaching test (concentrated ammonium nitrate solution as a leachant) coupled with SEM-EDS and DSC-TGA analyses. The effect of leaching on the physical and mechanical properties of such pastes was assessed and their chemical resistance to leaching was evaluated through a parametric model. It was found that the addition of nanosilica improves the chemical resistance of cement pastes by producing a significant leaching deceleration (up to 69%). As a result, the decay of compressive strength of leached specimens (about 60% reduction) is significantly retarded. Higher residual compressive strengths are also obtained for totally leached specimens containing nanosilica, as a result of the ability of this admixture of consuming portlandite during cement hydration and then reducing the porosity associated with portlandite dissolution during leaching.