The aim of this study was to determine the chemical composition and antioxidant activity of dandelion [Taraxacum officinale (Weber) ex Wigg.] and sea buckthorn (Hippophae rhamnoides Rousi) leaves grown in Poland (53°20′35″N, 14°58′10″E). Both the leaves of sea buckthorn and common dandelion were found to be a very good source of essential nutrients and elements. The dandelion leaves contained significantly more protein, fat, and crude fibre than those from sea buckthorn. Dandelion leaves also contained more phosphorus, potassium, calcium, iron, and zinc. Although the dandelion leaves were a good source of biologically active substances, sea buckthorn leaves exhibited a significantly higher level of antioxidant activity as measured by ABTS. Dandelion leaves were richer in tocopherols, thiamine, riboflavin, and niacin while the sea buckthorn leaves contained higher levels of l-ascorbic acid.