Arnica sororia is a species of arnica known by the common name twin arnica. It is native to western North America from British Columbia to California to Nebraska, where it grows in many types of habitat. It is a rhizomatous perennial herb producing one or more hairy, hairy stems 10 to 50 centimeters tall. There are several pairs of broadly lance-shaped leaves along along the stem, the lower ones borne on petioles. Leaves may reach up to 14 centimeters long. The flower cluster holds one to five daisylike flower heads lined with phyllaries coated in hairy hairs. The flower head has a center of hairy yellow disc florets and a fringe of yellow ray florets. The fruit is an achene a few millimeters long with a white pappus.