In this study, the opportunity to enhance the insecticidal activity of Rosmarinus officinalis essential oil was studied for effective management of the red flour beetle, Tribolium castaneum, as a stored product pest beetle. Nanoprecipitation method was used to prepare rosemary oil‐loaded nanocapsules. Bioassays were conducted at 27–30°C temperature and 70–75 % relative humidity in the dark. Fumigant toxicity of the non‐formulated oil and nanocapsules of R. officinalis were investigated at 13.20, 15.92, 19.12, 23.04, and 27.76 μL/L air after 24 and 72 h exposure and the contact toxicity of the non‐formulated oil and nanocapsules were investigated at 4.28, 3.55, 2.95, 2.45 and 2.36 μL/cm² after 24 h exposure. The major constituents of the essential oil of rosemary were α‐Pinene, 1,8‐cineol, camphor, and cis‐verbenone. Nanocapsules presented an average size (145 ± 15 nm) (± standard error [SE]) with a polydispersity index below 0.3, a negative zeta potential (−11.0 ± 0.5 mV), and a high encapsulation efficiency (78.20 ± 0.93 %). Scanning electron microscope photomicrograph of rosemary oil‐loaded nanocapsules showed the presence of spherical nanocapsules with regular and homogeneous surfaces. In fumigant and contact toxicity, there were significant differences between non‐formulated and rosemary oil‐loaded nanocapsules in all the concentrations and times. The results suggested that nanoencapsulated essential oils from R. officinalis can be used for effective control in T. castaneum. When this technique is used, it can produce pesticides that have controlled‐release properties and reduce the concentration of the applied doses and number of applications.