Development of new antimicrobial compounds against different microorganisms is becoming critically important, as infectious diseases are still one of the leading causes of death in the world. The pharmaceutical industry is searching for new lead compounds with novel chemical structures to overcome the increasing resistance to known antibiotics. Plants can be a useful source of these lead compounds. Xanthium cavanillesii Schouw, Asteraceae, grows wild in Uruguay and its infusion is used in traditional medicine as a skin antiseptic. We have previously reported studies on the antimicrobial activity of several extracts of X. cavanillesii against different microorganisms. In this work, we present the isolation and structural elucidation by spectroscopical methods of a sesquiterpene lactone with a new xanthanolide derived skeleton.