Development of synthesized methods of titanium dioxide, TiO2, known as an excellent photocatalyst material, is becoming more and more important due to the rapid growth of global market demands in sustainable and clean energy resources and water/air purification technologies. This high demand leads to focus on the development of synthesized methods for high-quality TiO2 with scalability. Gas-phase synthesis was demonstrated as a promising method for rapid production rate, which is suitable for industrial purposes. In this chapter, we will discuss the fundamental principles of gas-phase synthesis and recent progress in TiO2 production. General concepts of synthesized reactor setups and operations are presented with three types of gas-phase synthesis such as hot-wall reactor, flame reactor, and plasma reactor. The properties of TiO2 products such as morphological structures, sizes, phases, and compositions are tunable by simple operational parameters (e.g., precursor types and feeding methods, precursor concentrations and compositions, gas flow rates and gas compositions, reaction atmospheres and additives, and reaction temperatures). Furthermore, recent advancement of additive technologies demonstrates the more functional structures of TiO2 including heterostructures with other semiconductors and metal nanoparticles/clusters, and defected TiO2 by impurity doping can be synthesized within single step of these gas-phase methods.