This article reports a study of the nutritional composition, total dietary fiber (TDF), mineral contents, fatty acid and amino acid profiles, polyphenolic concentration and antioxidant activity of three Spanish seaweeds: two brown seaweeds (Himanthalia elongata and Undaria pinnatifida) and one red (Porphyra umbilicalis). TDF and ash were the most abundant components in the brown seaweeds, while TDF and protein were the main components in the red one. In all seaweeds, the lipid contents were very low but the polyunsaturated fatty acid contents were high. Although the red seaweed contained significantly (p < 0.05) more protein content than the brown ones, all three contained all the essential amino acids at levels comparable to FAO/WHO requirements. The brown seaweeds contained significantly more minerals than the red one; they are a rich source of K, Na, Ca and Mg and present a beneficial Na/K ratio. Himanthalia elongata displayed remarkably high polyphenolic content, endowing it with appreciable antioxidant activity. These seaweeds offer considerable potential as functional food ingredients due to components like dietary fiber, minerals and trace elements, protein and lipids, which produce many different kinds of biological activities, and also to their high antioxidant capacity.