Photocatalysis is being intensively studied for reactions such as water splitting and CO2 reduction, where absorption of light in a semiconductor such as TiO2 produces electrons and holes that can drive chemical reactions. Efficiencies on bulk materials, however, are too low to be of practical use. In recent years, low dimensional structures such as nanoclusters or nanotubes, displaying metal and oxygen coordination environments very different from the bulk, show promise for improved photocatalytic activities. Key to this is the presence of low coordinated metal and oxygen sites which can act as both charge carrier trapping sites and active sites with target molecules such as water or CO2. This paper presents the results of a density functional theory, with Hubbard U correction (DFT+U), study of electron and hole localization in free and metal oxide-supported TiO2 nanoclusters that display low coordinated titanium and oxygen sites. In free TiO2 nanoclusters, electrons and holes preferentially localize at 4-fold coordinated Ti and titanyl, i.e., singly coordinated, oxygen sites in TiO2. For TiO2 nanoclusters supported on rutile (110) electrons preferentially trap on a Ti site in the rutile surface and holes are trapped on the nanocluster–preferentially on titanyl oxygen, if present, and a 2-fold coordinated oxygen otherwise. Our analysis of La2O3 and SiO2 surfaces modified with a Ti5O10 nanocluster shows that electrons and holes are trapped on the TiO2 nanocluster, with electrons residing on a low coordinated Ti site at the interface between TiO2 and the support. Holes are trapped on low coordinated oxygen sites. An important finding is that the modification of the support oxides with these TiO2 nanoclusters induces a band gap narrowing over the unmodified oxide, which will induce a red shift in light absorption. Overall, these studies provide important insights for the design of improved photocatalysts, in that we should seek structures with low coordinated metal and/or oxygen sites that can trap electrons and holes and be useful in adsorbing and activating molecules of interest in photocatalysis.