In this study, the antioxidant and cytotoxic activity, the phenolic content and the HPLC-DAD/ESI-MS profile of the ethanolic extracts of five Amaranthaceae plants collected in the south of Brazil were investigated: these were Alternanthera philoxeroides (Mart.) Griseb. (I), Alternanthera hirtula (Mart.) R.E. Fr. (II), Alternanthera praelonga A. St.-Hil. (III), Froelichia tomentosa (C. Mart.) Moq. (IV) and Pfaffia tuberosa (Spreng.) Hicken (V). The antioxidant potential was determined using the DPPH and ORAC-FL assays. The total phenolic content was measured using the Folin-Ciocalteu reagent. The ethanolic extracts of I, II, IV and V showed high levels of phenolic compounds (3.6-20.0 mg GAE/Kg), as well as high antioxidant activity in both methods. The cytotoxicity of the extracts was investigated in vitro against panel of human cancer cell lines and against VERO control. The extract of V exhibited anti-proliferative activity against all cancer cell lines studied (Total Growth Inhibition, TGI<100 µg/mL), except for VERO cell line (TGI=125.2 µg/ mL). HPLC-UV/DAD and ESI-MS analyses revealed that the extracts investigated appear to contain phenolic acids and flavonoids as main constituents. Findings from this study demonstrated that the extracts of the Amaranthaceae plants from the south of Brazil may be considered as promising sources of antioxidant and anti-proliferative compounds.