Oxidative stress is the outcome of the difference between antioxidants and pro-oxidants in an organism, and it is vital in the pathogenesis of several degenerative disorders. The use of natural antioxidants is effective in delaying the oxidation of biomolecules and thus preventing diseases. The present study aimed to investigate the phytochemical screening and in vitro antioxidant activity of Sphaeranthus indicus leaf extract. Phytochemical screening of Sphaeranthus indicus leaves showed the presence of tannin, saponin, flavonoids, steroids, terpenoids, triterpenoids, alkaloids, anthraquinones, polyphenol, glycoside, coumarins while emodins and anthocyanins were absent. GC-MS analysis was performed to identify the bioactive chemical constituents present in the extract. Twenty compounds were identified in the leaf extract of Sphaeranthus indicus by GC-MS analysis. Sphaeranthus indicus leaf extract showed significant antioxidant activity against free radical scavenging by DPPH, total antioxidant, iron chelator, hydroxyl radical scavenging, and nitric oxide scavenging activity. The effects were comparable to those of the standard antioxidant ascorbic acid and these antioxidant properties were concentration dependent. This study recommends the biological assay of extracts against disorders related to oxidative stress for the development of phytomedicine with antioxidant properties. These antioxidant activities could be due to the presence of antioxidant phytochemicals such as flavonoids, phenols, terpenoids and saponins, among others.