Ageratina occidentalis is a species of flowering plant in the daisy family known by the common name western snakeroot. It is native to the western United States where it grows in several types of habitat. This is a rhizomatous perennial herb growing fuzzy green or purple stems to a maximum height near 70 centimeters. The hairy leaves are triangular in shape with serrated edges. The flower cluster is a dense cluster of fuzzy flower heads containing long, protruding disc florets in shades of white, pink, and blue. There are no ray florets. The fruit is an achene a few millimeters long with a rough, bristly pappus.