We report a one-pot protocol to prepare ultrathin nanosheets of brookite-phase TiO2 through hydrolyzing TiCl3 in formamide. This 2D titania is defective and shows flexible electronic states and enhanced surface reactivity, which is probed by H2O2 adsorption, catalytic TMB oxidation, and X-ray absorption fine structure. The nanosheets provide a new 2D platform to exploit the applications of TiO2 in catalysis, energy conversion and storage.