Sea fennel (Crithmum maritimum L.) is a perennial halophytic plant that grows along the entire Adriatic coast in Croatia. The plant has been known since ancient times and the leaves are consumed as food and spice, while in folk medicine it is used as a medicine. Due to the numerous biologically active substances present throughout the plant, various researches have been carried out in order to gain new knowledge and make better use of the plant itself. The aim of this work was therefore to investigate the chemical composition and antioxidant activity in order to compare the differences between the flower and leaf extracts. Samples were collected from three different locations: on the island of Pag, on the island of Korčula and in Cavtat. The plant extracts were used to determine the content of total phenols and flavonoids, while the concetration of chlorogenic acid was determined by high-performance liquid chromatography (HPLC). Three methods are used to determine antioxidant activity: ferric reducing antioxidant power (FRAP), the method of reduction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the oxygen radical absorbance capacity (ORAC). From the obtained results, it can be concluded that the extracts of sea fennel leaves and flowers have a high content of total phenols, flavonoids and chlorogenic acid. Besides the high content of phenolic components, the extracts also showed very good antioxidant activity, which was determined by all methods. In the study, the influence of the location of harvesting of the sea fennel was also observed, so that the samples from Korčula showed the highest content of phenolic components and the best antioxidant activity, while the samples from Pag showed the lowest results. In addition, the extracts from the flowers of sea fennel showed better results than the leaf extracts, indicating the extraordinary potential of this plant, which has not yet been sufficiently researched and exploited.