Hijiki (ヒジキ, 鹿尾菜 or 羊栖菜, hijiki) (Sargassum fusiforme, syn. Hizikia fusiformis), sometimes called hiziki, is a brown sea vegetable growing wild on rocky coastlines of East Asia.

Hijiki has been a part of the Japanese culinary sphere and diet for centuries. It is rich in dietary fiber and essential minerals such as calcium, iron, and magnesium. According to Japanese folklore, hijiki aids health and beauty, and thick, black, lustrous hair is connected to regular consumption of small amounts. Hijiki has been sold in United Kingdom natural products stores for 30 years and its culinary uses have been adopted in North America.

Recent studies have shown that hijiki contains potentially toxic quantities of inorganic arsenic, and the food safety agencies of several countries (excluding Japan), including Canada, the United Kingdom, and the United States, have advised against its consumption.