In this study, the effect of TiO2 nanoparticles (NP) was evaluated on the essential oil of rosemary plant (Rosmarinus officinalis). The experimental treatment included spraying seven doses of TiO2 NPs (0 (control), 20, 40, 60, 100, 200, and 400 ppm) on the rosemary leaf. The results indicated that the use of TiO2 NPs significantly affected the quantity of rosemary essential oil. The peak area for most of the compounds increased with the application of TiO2 NPs. However, at high concentrations of TiO2 NPs (more than 200 ppm), the peak area decreased. The peak area of some compounds, such as myrecene, 2-butenal, 2-ethenyl-, 3-pinanone, isoborneol, and β-terpineol was observed in different conditions, and a significant increase in concentration was observed in the groups that received 60 or 100 ppm TiO2 dose. This study was the first to evaluate the favorable effects of TiO2 NPs on the essential oil of a highly valuable medicinal plant.