The effectiveness of nano-silica (NS) in preventing deleterious expansion associated with alkali-silica reaction (ASR) was assessed using concrete mixes made with two siliceous aggregates of different alkali-reactivity. Concrete mixes were also made using condensed silica fume as a reference mineral admixture. Two different expansion tests were used: the accelerated expansion test at 60°C and 100% RH and the ultra-accelerated expansion test in alkaline solutions at 150°C. Irrespective of the expansion test used, nano-silica proves to be effective as an ASR inhibitor only when the driving force for ASR expansion, represented by the difference between the alkali content, Lac, of the concrete and the Threshold Alkali Level, TAL, of the aggregate, does not exceed 1.2 kg Na2Oeq/m3. Conversely, condensed silica fume is always effective at least up to the ASR driving force of 2.2 kg Na2Oeq/m3, and no counterproductive effect on ASR prevention is observed.