Reynoutria japonica, commonly known as Japanese knotweed or itadori, is a perennial herbaceous plant in the family Polygonaceae. It produces tall, bamboo-like stems and broad, heart-shaped leaves, and is well known both for its resilience and its high concentration of bioactive compounds. The genus name Reynoutria honors Jean Reynou, a 17th-century French botanist, while the species epithet japonica refers to its origin in Japan. The plant was formerly classified under Polygonum and Fallopia, reflecting its complex taxonomic history.

Native to East Asia, particularly Japan, China, and Korea, R. japonica has been used since ancient times in traditional medicine—especially in Japanese kampo and Chinese herbal medicine—as an anti-inflammatory, diuretic, and cardioprotective herb. It was also valued as a wild edible plant, with young shoots eaten as a tart spring vegetable similar to rhubarb. In modern times, Japanese knotweed has become both a source of valuable phytochemicals and an invasive species in many temperate regions worldwide. Industrially, it is cultivated as a primary natural source of resveratrol, an antioxidant compound studied for its anti-aging, cardioprotective, and neuroprotective effects. Extracts are used in nutraceuticals, skincare, and functional foods.