Polygonum pensylvanicum (syn. Persicaria pensylvanica) is a species of flowering plant in the buckwheat family, Polygonaceae.
Pennsylvania smartweed is a variable annual herb reaching 10 cm (3.9 in) to 2 m (6.6 ft) tall. The upright, ribbed stems are branching or unbranched. The lance-shaped leaves reach up to about 2 cm (0.79 in) in length. The blade may be marked with a dark blotch. The brownish ochrea at the base is up to 2 cm (0.79 in). The inflorescences grow at the top of the stem and from the leaf axils. The flowers have five pinkish or greenish tepals each a few millimeters long.
This plant grows in moist, disturbed habitat types, such as ponds, reservoirs, riverbanks, irrigated fields, and ditches.