Seed production plays an essential role in the life cycle of angiosperms. Within the Brassicaceae family, which includes species that have successfully colonized a wide range of habitats, seed maturation has been extensively studied in the model plant Arabidopsis thaliana and several agronomically important species. In this study, we investigate this developmental process in Cakile maritima, a wild halophytic member of the Brassicaceae that thrives along sandy coastlines from the Arctic to the Mediterranean. This species is also widely used as a model for studying salt tolerance mechanisms. By independently analyzing the different zygotic tissues of the seed, namely, the embryo and the endosperm, we demonstrate that, quantitatively, the embryo is the main site of storage compound accumulation. Notably, the biochemical composition of these reserves differs substantially between the zygotic tissues. Approximately half of the fatty acids in the endosperm consist of omega-7 monounsaturated fatty acids, which are largely absent in the embryo. By contrast, the embryo shows a stronger induction of omega-9 and polyunsaturated fatty acid biosynthesis pathways compared to the endosperm, reflecting tissue-specific regulation of fatty acid metabolism genes. Furthermore, seeds collected from different ecological niches along a latitudinal gradient reveal that environmental temperature significantly affects the composition of seed reserves, particularly the proportion of polyunsaturated fatty acids in the embryo. Together, these findings underscore the metabolic diversity and adaptative potential of C. maritima, providing valuable insights into seed development, environmental plasticity, and oil composition within the Brassicaceae.