Polygonum douglasii is a species of flowering plant in the knotweed family known by the common name Douglas's knotweed. It is native to much of northern and western North America, where it can be found in many types of habitat, including disturbed areas. It has been found in Canada from British Columbia north to Yukon and east as far as Québec, and in the United States as far south as California, New Mexico, Iowa, and New York. Polygonum douglasii is known as a species complex, as there are many subspecies that may be better treated as species in their own right.