Seaweeds cover a wide range of applications, e.g. as food supplements, in animal feed, as biofuels or as sources of bioactive compounds. The Greek coast in the East Mediterranean is rich in various seaweeds that remain unexploited because their chemical and nutritional content has not yet been characterized. In the present study, eight seaweeds belonging to the Rhodophyta, Ochrophyta (class Phaeophyceae) and Chlorophyta were biochemically characterized and evaluated as potential food sources. Total polyphenol content, antioxidant activity, fatty acids and elemental composition were measured. Acanthophora nayadiformis, Ceramium sp. (Rhodophyta) , Codium fragile (Chlorophyta) , Cystoseira foeniculacea and Gongolaria barbata (formerly Cystoseira barbata ) (Ochrophyta, Phaeophyceae) had the highest phenolic content and strongest antioxidant activity. Both brown and red seaweeds were rich in minerals, with G. barbata, Dictyopteris polypodioides (formerly Dictyopteris membranacea ) (Chlorophyta, Phaeophyceae) and A. nayadiformis being the richest in macro- and microelements. The low Na/K ratio in most seaweeds (0.03–3.49) and the high iron content of red and brown algae (1.01–52.40 mg 100 g −1 of wet tissue) make algal consumption an attractive option. Chlorophyta and Phaeophyceae had the lowest n-6/n-3 PUFA ratios, with α-linolenic acid being the most abundant n-3 PUFA. The green algae Codium fragile and Ulva lactuca had the highest oleic and docosahexaenoic acid content, respectively. Finally, Rhodophyta were the highest producers of eicosapentaenoic acid. The findings confirmed the nutritional value of all seaweeds, highlighting brown seaweeds Cystoseira foeniculacea , G. barbata , and D. polypodioides as potential sources for food supplements and candidate species for seaweed cultivation in Mediterranean coastal waters.