Nasturtium officinale W. T Aiton or “watercress” (Brassicaceae) is a nutritionally valuable plant that is the richest source of carotenoids, polyphenols, iron, calcium, iodine, folic acid, essential vitamins and minerals. It is traditionally used for their appetiser, anti-scorbutic, stimulant, diuretic and detoxifying properties. The present study was conducted to investigate the antioxidant potential, phytochemical contents (total phenolic, flavonoids and tannin contents) as well as a preliminary phytochemical screening of different extract of aerial parts of Nasturtium officinale collected from the Kumaun region. The different extracts showed significant antioxidant activity as well as total phenolic, flavonoids and tannin contents. The preliminary phytochemical screening showed the presence of several phytochemical constituents such as carbohydrates, proteins, amino acids, glycosides, alkaloids and others. The hydroalcoholic extract possess significantly (P<0.05) higher antioxidant potential with IC50 value (0.333 mg/ml, 0.509mg/ml)and EC50 value (3.537 mg/ml) by 2,2-diphenyl-1-picrylhydrazyl(DPPH), 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid)(ABTS), and ferric-reducing antioxidant power (FRAP) methods, respectively, as well as highest total tannin, flavonoid and phenolic contents among all the extracts. These results provide substantial evidence that the plant Nasturtium officinalehas antioxidant potential and valuable sources of phytochemicals.