The antioxidant (DPPH, superoxide anion radical scavenging and FRAP) and antiproliferative activities against Vero cells (african green monkey kidney), C6 cells (rat brain tumor cells) and HeLa cells (human uterus carcinoma) of Cirsium arvense (CA) of all parts [root (Kinghorn et al. 2004), stem (CAS), flower (CAF) and leaves] and isolated compounds were investigated in vitro. CAR was shown to have the highest antiproliferative activity against HeLa and C6 cell lines as compared to other parts. A flavon glucoside (linarin), a steroid glucoside (daucosterin), a steroid (stigmasterol) and a new benzymidazole molecule (E−6-(3-(5,6-dimethoxy-1H-benzo[d] imidazol-2-yl)allyloxy)−5-(hydroxymethyl)-tetrahydro-2H-pyran-2,3,4-triol) were isolated from the CAR. The structures of these molecules were determined by the 13C (carbon) and 1H (proton)- Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR), and Gas Chromatography-Mass Spectrometry (GC-MS) techniques. CAR for C6 cell line, CAS for HeLa and Vero cell line were observed the highest antiproliferative activity. In addition, CAR showed the stronger DPPH activity than α-tocopherol and BHT at high concentrations. The FRAP and superoxide anion radical scavenging activities of CAR (Fr-7) were found to be higher than BHT. Practical Applications: Cirsium species are considered as agricultural weed among villagers in Turkey and are used as alternative medicinal plant for years. Studies have shown that these species are rich with regard to bioactive seconder metabolites. There is no phytochemical/bioactivity study about this plant until now. Our findings showed that the extracts of this plant possessed potent antioxidant activities. Four compounds, one of which is a new one to literature, were isolated and characterized. Finally, Cirsium arvense, being considered as an invader plant in agricultural fields, can be evaluated as a good source of antioxidant products.