Arnica sororia is a North American species of arnica known by the common name twin arnica. It is native to western Canada (British Columbia, Alberta, Saskatchewan) and the western United States (Washington, Oregon, California, Nevada, Utah, Idaho, Montana, Wyoming, South Dakota). It grows in grasslands and in conifer forests. Arnica sororia is a rhizomatous perennial herb producing one or more hairy, glandular stems 10 to 50 centimeters tall. There are several pairs of broadly lance-shaped leaves along the stem, the lower ones borne on petioles. Leaves may reach up to 14 centimeters long.