Hierarchical macro-/mesoporous titania is prepared without the addition of templates or auxiliary additives at room temperature by the simple dropwise addition of tetrabutyl titanate to pure water, and then calcined at various temperatures. The products are characterized by X-ray diffraction, N2-adsorption–desorption analysis, scanning electron microscopy, and the corresponding photocatalytic activity is evaluated by measuring the photocatalytic oxidation of acetone in air. The results reveal that hierarchical macro-/mesoporous structures of titania can spontaneously form by self-assembly in alkoxide–water solutions in the absence of organic templates or auxiliary additives. The calcination temperature has a strong effect on the structures and photocatalytic activity of the prepared titania. At 300 °C, the calcined sample shows the highest photocatalytic activity. At 400 and 500 °C, the photocatalytic activity slightly decreases. When the calcination temperature is higher than 500 °C, the photocatalytic activity greatly decreases because of the destruction of the hierarchical macro-/mesoporous structure of the titania and the drastic decrease of specific surface area. The hierarchically macro-/mesostructured titania network with open and accessible pores is well-preserved after calcination at 500 °C, indicating especially high thermal stability. The macroporous channel structures are even preserved after calcination at 800 °C. This hierarchical macro-/mesostructured titania is significant because of its potential applications in photocatalysis, catalysis, solar-cell, separation, and purification processes.