Persicaria longiseta is a species of flowering plant in the knotweed family known by the common names Oriental lady's thumb, bristly lady's thumb, Asiatic smartweed, long-bristled smartweed, Asiatic waterpepper, bristled knotweed, bunchy knotweed, and tufted knotweed. It is native to Asia (China, India, Russia, Japan, Malaysia, etc.), and it is present in North America and Europe as an introduced species and often a weed.