Anatase TiO2 is a potential negative electrode for sodium-ion batteries. The sodium storage mechanism is, however, still under debate, yet its comprehension is required to optimize the electrochemical properties. To clarify the sodium storage mechanism occurring in anatase, we have used both electrochemical and chemical routes from which we obtained similar trends. During the first discharge, an irreversible plateau region is observed which corresponds to the insertion of Na + within the interstitial sites of anatase and is accompanied by a drastic loss of the long-range order as revealed by x-ray diffraction, high resolution of high angle annular dark field scanning transmission electron microscope (HAADF-STEM) and pair distribution function (PDF) analysis. Further structural analysis of the total scattering data indicates that the sodiated phase displays a layered-like rhombohedral R-3m structure built from the stacking of Ti and Na slabs. Because of the initial 3D network of anatase, the reduced phase shows strong disorder due to cationic inter-mixing between the Ti and Na slabs and the refined chemical formula is (Na0.43Ti0.57)3a(0.22Na0.39Ti0.39)3bO2 where refers to vacancy. The presence of high valence Ti ions in the Na layers induces a contraction of the c-parameter as compared to the ordered phase. Upon de-sodiation, the structure further amorphized and the local structure probed by PDF is shown to be similar to the anatase TiO2 suggesting that the 3D network is recovered. The reversible sodium insertion/de-insertion is thus attributed to the rhombohedral active phase formed during the first discharge, and an oxidized phase featuring the local structure of anatase. Due to the amorphous nature of the two phases, the potential-composition curves are characterized by a sloping curve. Finally, a comparison between the intercalation of lithium and sodium into anatase TiO2 performed by DFT calculations confirmed that for the sodiated phase, the rhombohedral structure is more stable than the tetragonal phase observed during the lithiation of nanoparticles. In many areas of modern life, lithium-ion batteries are ubiquitous as energy-storage solutions. The growing demand for higher energy density and lower cost of electro-chemical energy storage devices, however, has motivated a search for auxiliary technologies based on alternative chemistries. 1,2 One possible candidate is the sodium-ion battery, which is attractive because of the high earth– abundance of sodium, and lower cost versus lithium-ion batteries, due to compatibility with aluminum as the an-odic current collector. 3-5 Development of sodium-ion batteries has been largely stimulated by knowledge of lithium-ion analogues. The intercalation of Na + or Li + ions into a host lattice can, however, give qualitatively different voltage profiles, corresponding to different intercalation mechanisms. For example, lithium insertion in Li4Ti5O12 is accompanied by a spinel to rock-salt phase transition. 6,7 The equivalent sodium insertion, however, proceeds via a complex three-phase–separation mechanism (spinel to two rock-salt phases of Li7Ti5O12 and Na6LiTi5O12). 8 Such differences in intercalation behaviour can often be attributed to different properties of Li versus Na, such as ionic radius and polarizability. 9, 10 In general, however, the performance of electrodes in sodium-ion batteries cannot be understood by simply extrapolating from their behaviour versus lithium, when it is necessary to carefully reexamine the sodium-intercalation behaviour.