Juice squeezed from cereal grasses harvested at the jointing stage, i.e., wheatgrass (Triticum aestivum), exhibits high antioxidant activity. Rice (Oryza sativa) may also exhibit antioxidant activity. We therefore examined the antioxidant activity of juices squeezed from grasses harvested at the jointing stage for seven coloured and seven white Thai rice cultivars. The antioxidant activity was determined using 2,2-diphenyl-1-picrylhydrazyl, ferric reducing antioxidant power, β-carotene bleaching, and thiobarbituric acid reactive substances assays. The total phenolic content (TPC) and total monomeric anthocyanin content (TMAC) were also determined. Coloured (purple) rice grass juices exhibited greater antioxidant potential than the grass juices from white rice and wheat. The coloured rice cultivar Kum Doisaket exhibited the highest antioxidant activity in all assays. Correlation analysis indicated that the TPC and TMAC could be responsible for the antioxidant activity. The DNA protective properties of the coloured rice cultivars Kum Doisaket and Kum Noi and wheat were also examined. Only the Kum Doisaket cultivar exhibited a dose-dependent DNA protective effect. The notable antioxidant efficacy for the Kum Doisaket cultivar may be influenced by the high level of anthocyanins present in its grass juice. This finding suggests the possibility of developing functional foods from coloured rice grass.