Persicaria wallichii is a species of flowering plant in the knotweed family known by the common names Himalayan knotweed and cultivated knotweed.

Persicaria wallichii is native to Asia (southwestern China, Indian Subcontinent, Myanmar, Afghanistan) and it is planted many places as an ornamental. It has the capacity to become an invasive species and has established itself in some parts of North America, primarily the Pacific Coast of the United States and Canada.