Assessment of the dynamics of rosemary (Rosmarinus officinalis) is essential in the production of essential oils (EOs) in Morocco, considering the country is one of the main producers of rosemary EO. In this study, we aimed to examine the influence of harvesting period and environmental factors on the dynamics of rosemary EO, mainly its composition. Samples were collected from the four sites in northeastern Morocco on a monthly basis between July 2021 and June 2022. Subsequently, quantitative and qualitative analyses by hydrodistillation and gas chromatography were performed to determine the yield and composition of EOs. On average, EO yields ranged from 2.3 to 3% across the four sites and were highest in summer and lowest in autumn. A moderate negative correlation (r = -.59, p < .05) was observed between precipitation and EO yield, while temperature had a moderately positive influence. A total of 17 chemical compounds, representing 88.9-99.1% of the EO extracts, were identified and consisted mainly of 1,8-cineole (44.2-46.6%), camphor (14.8-16.8%), borneol (7.5-9.1%), and α-pinene (5.2-5.9%). Harvesting period strongly influenced EO composition, with the highest concentrations of 1,8-cineole and α-pinene were recorded during the summer period (July and August), while the concentrations of borneol, camphor, and terpineol were highest in winter (December and January) and late spring. The findings of the study highlight the importance of monitoring the factors that influence the chemical composition of rosemary EO, thus providing a knowledge base that would help improve the quality and economic value of rosemary EO production in the region.