The biochemical compositions and nutritive values of six species of seaweeds were analyzed to determine their applicability in functional foods or ingredients. The biochemical compositions (moisture, ash, protein, lipid, and dietary fiber) and fatty acid contents were determined for the following seaweed extracts: Phaeophyceae (Laminaria japonica, Hizikia fusiformis, and Undaria pinnatifida), Rhodophyceae (Porphyra tenera and Gracilaria verrucosa), and Chlorophyceae (Ulva lactuca). The moisture content (% dry weight) ranged from 11.47% to 13.94%, ash from 19.15% to 26.50%, protein from 5.08% to 15.44%, lipid from 2.75% to 4.43%, and dietary fiber from 36.84% to 52.98%. C14:0, C16:0, C18:0, C16:1, C18: 1n-3, C18:2n-6, C18:3n-6, C20:4n-6, and C20:5n-3 represented the predominant proportions of fatty acids. Interestingly, docosahexaenoic acid (C22:6n-3, DHA) was either not found or only detected in trace amounts in the analyzed seaweeds. The levels of n-3 fatty acid were higher than other polyunsaturated fatty acids, and the n-6/n-3 ratio was very low. These results indicate that seaweed inhabiting Korean coastal areas will be beneficial to human health.