Polygala senega is a species of flowering plant in the milkwort family, Polygalaceae. It is native to North America, where it is distributed in southern Canada and the central and eastern United States. Its common names include Seneca Snakeroot, Senega Snakeroot, Senegaroot, Rattlesnake Root, and Mountain Flax. This plant had many uses among Native Americans. This species is a perennial herb with multiple stems up to 50 centimeters tall. A mature plant can have up to 70 stems growing from a hard, woody rootstock that spreads horizontally. The inflorescence is a spike of rounded white or greenish flowers. The fruit is a capsule containing two hairy black seeds. The plant grows on prairies and in woods and wet shoreline and riverbank habitat.