The lithium insertion behavior of nanoparticle (3-D) and nanosheet (2-D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments. DFT+U calculations show unique lithiation mechanisms for the different nanoarchitectures. For TiO2(B) nanoparticles, A2 sites near equatorial TiO6 octahedra are filled first, followed by Al sites near axial TiO6 octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and Al. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li+-Li+ interactions between C and A2 sites. The calculated lithiation potentials and degree of filling agree qualitatively with the experimentally observed differential capacity plots.