The chemical composition and antioxidant activity of the essential oil from the leaves of Ambrosia arborescens Miller grown in the Peruvian Andes were studied. One hundred and thirteen compounds were identified, which represents 94.7% of the total composition of the essential oil. Sesquiterpene hydrocarbons and oxygenated monoterpenes were the most represented classes of volatiles with 43.8% and 22.4%, respectively. The main constituents of the essential oil were β-acoradiene (15.3%) and chrysanthenone (11.3%). Antioxidant activities were examined using 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP) and determination of lipid peroxidation in brain rat homogenates (LPO) assays. The essential oil had a free radical scavenging effect, and it displayed ferric reducing capacity and antioxidant activity by inhibition of LPO.