Seasonal variation in the nutrient composition of Mytilus coruscus (thick shell mussel), cultivated in Shengsi Islands, Zhejiang Province, China, was investigated in this study. Proximate composition, mineral and amino acid concentrations, as well as the compositions of the lipid and fatty acid of thick shell mussels were analyzed. Proximate composition and mineral contents were seasonally varied significantly except for calcium (Ca) and lead (Pb). Glycine was the predominant amino acid in mussels throughout all seasons, while lysine, threonine, phenylalanine, and arginine were the main essential amino acids. The predominant lipids were phospholipids, followed by triacylglycerols and sterol esters. Polyunsaturated fatty acids (PUFA) predominated over saturated and monounsaturated fatty acids throughout the year. Docosahexaenoic acid (12.44-18.34% of total fatty acids) and eicosapentaenoic acid (10.79-14.60%) were the most abundant PUFA. Significant seasonal variations were observed in the compositions of most amino acids, lipid classes, and fatty acids. Cultivated Chinese thick shell mussels represent a source of the health benefiting long chain n-3 PUFA, essential amino acids, and minerals for human consumption.