Eryngium aquaticum is a species of flowering plant in the family Apiaceae known by the common name rattlesnakemaster, marsh rattlesnake master, corn-snakeroot, bitter snakeroot, and marsh eryngo. This plant is native to eastern North America. This biennial or perennial herb grows up to 2 meters tall. The ribbed, erect stem branches toward the top. There are alternately arranged leaves which are lance-shaped and toothed on the edges. The basal leaves may be up to 90 centimeters long by 9 wide. The inflorescence contains white to blue flower heads with spiny, blue-tinged bracts.