Porphyra yezoensis is potentially an excellent source of dietary fiber, amino acids, and eicosapentaenoic acid (EPA) because this red seaweed is available in large quantities and is rich in polysaccharides, proteins, and n-3 fatty acids. This study determined the insoluble dietary fiber (IDF), soluble dietary fiber (SDF), neutral detergent fiber (NDF), acid detergent fiber (ADF), amino acid, and fatty acid contents of P. yezoensis harvested monthly from November 2011 to March 2012. The total dietary fiber (TDF) and IDF contents ranged from 27.2-34.9% and 18.5-26.9%, respectively, and were greater in March than November. The SDF content ranged from 4.9-8.4% and did not differ significantly during growth. Galactose and 3,6-anhydro galactose were the major sugars in IDF and SDF. The higher levels of galactose and 3,6-anhydro galactose in IDF might be due to associated porphyran-type polysaccharides. Mannose and xylose were also major sugars in IDF. The total amino acid contents decreased gradually from November to March. The total amino acid composition of Porphyra was dominated by alanine, glutamic acid, arginine, and aspartic acid. No significant changes in the fatty acid profile were observed throughout the study period. The dominant fatty acid during all seasons was EPA, which comprised as much as 50% of the total fatty acid content.