The species Alternanthera brasiliana (L.) Kuntze (Amaranthaceae) is a perennial herbaceous plant widely distributed in Brazil's eastern beaches and certain Amazonian regions. Crude ethanolic (70%) extract, fractions (dichloromethane, ethyl acetate and n-butanolic) and the mixture isolated were investigated regarding their content of total phenolic compounds by Folin-Ciocalteau assay. Total phenolic contents varied from 29.0 to 56.6 mg/g (dry fraction). The scavenging capacity of the extract, fractions and mixture isolated was characterised by the 1,1-diphenyl-2-picrylhydrazyl method. Only ethyl acetate fraction exhibited a modest scavenging capacity (SC50 = 163.00 μg/mL and SC% = 64.29% at 250 μg/mL) in a dose-dependent manner. Crude extract, dichloromethane and butanolic fractions presented a very weak scavenging capacity (SC% ≤ 40% at 250 μg/mL). A correlation between the antioxidant activity and the total phenolic contents could not be observed. The fractionation and purification of dichloromethane fraction evidenced the presence of a mixture of β-sitosterol, stigmasterol and spinasterol, which was identified by NMR methods and compared with literature data. Results of this study demonstrated for the first time the antioxidant activity of A. brasiliana.