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Simple Summary: This study was conducted to investigate whether the positive impact of elevated CO2 (eCO2) on the chemical composition of aniseed (Pimpinella anisum L.) seeds is dependent on seed developmental stages and origin. To this end, we investigated the biochemical changes in eCO2-treated aniseed accessions from Tunisia, Syria, Turkey, Morocco, Yemen, and Egypt during three developmental stages (immature, premature, and mature). The highest dry weight percentages and seed yields were recorded for the Egypt and Morocco accessions. eCO2 has inducing properties on the nutritive and biological values of aniseeds, yet its effectiveness is related to seed maturity and provenances. For instance, seed maturation increased the nutrients and antioxidant metabolites in most eCO2-treated accessions. Conversely, essential oil metabolism was decreased by seed maturation but this effect was significantly reduced by the use of eCO2. The enhanced accumulation of bioactive compounds in eCO2-treated seeds was accompanied by improved health benefits. In this regard, eCO2 induces the antioxidant and hypocholesterolemic activities of aniseeds, particularly at mature stages. Thus, the present study confirms that there are significant interactions between eCO2 exposure, aniseed maturity, and origin on the chemical composition and pharmaceutical properties of aniseed. Besides the lack of studies regarding applying elevated CO2 (eCO2) as a strategy to improve the chemical composition of anise (Pimpinella anisum L.) seeds, studies on its interaction with seed developmental stages and origin are very limited. The seed yield, chemical composition, and biological activity of 6 aniseed accessions (Egypt, Tunisia, Syria, Turkey, Yemen, and Morocco) were investigated during three developmental stages (immature, premature, and mature) under control and elevated CO2 conditions. Mature seeds from all aniseed accessions had significantly higher (p < 0.05) dry weight (DW) percentages than premature and immature seeds. The highest DW percentages were recorded in Egypt and Morocco accessions. Seed maturation increased nutrients and antioxidant metabolites in most eCO2-treated accessions. In contrast, essential oils were decreased by seed maturation, while eCO2 reversed this effect. Essential oil-related precursors (e.g., phenylalanine) and enzyme activities (3-Deoxy-d-arabino-heptulosonate-7-phosphate synthase (DAHPS) and O–methyltransferase) decreased with seed maturity. However, high CO2 reduced this impact and further induced the other essential oil-related precursors (shikimic and cinnamic acids). Consequently, eCO2 provoked changes in the antioxidant and hypocholesterolemic activities of aniseeds, particularly at mature stages. Overall, eCO2 application, as an efficient way to improve aniseed growth, essential oil metabolism, and chemical composition, was affected by seed maturation and origin. Future studies of eCO2-treated aniseeds as a nutraceutical and pharmaceutical product are suggested. [ABSTRACT FROM AUTHOR]