This work presents a novel chemical vapor deposition (CVD) approach that enables the deposition of ultrathin and conformal SiO2 layers on TiO2 anatase nanoparticles at room temperature using SiCl4 and air containing water without the use of a catalyst. The morphology of the CVD-grown SiO2 layers was found to be strongly dependent on the initial surface states of the TiO2 nanopowders, which could be altered by applying a simple heat pretreatment. The deposition on untreated TiO2 resulted in granular films, whereas on preheated TiO2 highly uniform and conformal SiO2 layers were obtained. By varying the SiCl4 precursor dosing time and the number of CVD cycles, the thickness of the SiO2 could be controlled at the nanometer level, which allowed us to investigate the influence of film thickness on the photocatalytic suppression ability. We found that a conformal SiO2 layer with a thickness of 3 nm could sufficiently suppress the photocatalytic activity of anatase TiO2 nanoparticles, which was demonstrated by the photodegradation of Rhodamine B. Our approach offers a simple, fast, feasible and low-temperature deposition method which can be directly applied to SiO2 coating on nanoparticles in pigments and other fields, particularly heat-sensitive materials, and further developed for large-scale production.