We show with DFT calculations that hydroxyl formation on anatase TiO2 from dissociation of water molecules is strongly enhanced by a nanostructured state of the oxide, involving the occurrence of complexes consisting of undercoordinated 4-fold Ti-2-fold O pairs. Our study of ridges delimited by anatase TiO2 (101) surfaces of different orientations shows that hydroxyl formation is significantly increased by the high acidity/basicity combination of these Ti-4c-O-2c pairs located on the ridges and by a stabilization effect associated with chemical bonds of the hydrogen atoms. This is at variance with the cases of the ideal (101) surface and of a ridge with 5-fold coordinated Ti atoms, where water adsorption is molecular. We demonstrate that these pairs of undercoordinated atoms on ridge edges have a particularly strong reactivity, and we propose that they play a major role in the observed high chemical activity of TiO2 nanosystems.