First-principles calculations suggest that doping TiO2 with carbonate can effectively reduce the bandgap of TiO2, thus making TiO2 photoactive in the visible region of the solar spectrum. Herein we report a simple “one-pot” solvothermal method for synthesizing brown carbonate-doped TiO2 microspheres. The diameter of the TiO2 microsphere is tunable from 0.5 to 4 μm with the nanopore size in the range of 3–11 nm. Remarkably, the specific surface area of these nanoporous anatase TiO2 microspheres can be as high as 500 m2 g−1. A controllable amount of carbonate could be incorporated into TiO2 through low-temperature post-synthesis annealing, which extends the light absorption of the TiO2 microspheres from the ultraviolet to the visible region of the solar spectrum. Very high photocatalytic activity of these carbonate-doped TiO2 microspheres was demonstrated in the visible light region for both photocatalytic production of hydrogen from water and degradation of methyl orange. Under 3 Sun visible-light illumination (λ ≥ 400 nm), the carbonate-doped TiO2 microspheres can produce 0.2 mmol H2 h−1 g−1 of photocatalyst, which is significantly higher than those from various other TiO2 photocatalysts.