Abstract Background Commelina benghalensis Linn. (Family: Commelinaceae) is a common weed available in Bangladesh with several uses in traditional medicine. However, the chemical profile of this medicinal plant is scarce in relation to its medicinal uses. The aerial parts of this plant have been investigated for the isolation of secondary metabolites and evaluation of the biological activities. Methods Major phytochemical groups were analyzed using chromogenic reagents, whereas n-hexane soluble fractionates of the methanol extract were subjected to ¹H nuclear magnetic resonance (NMR) spectroscopic analysis. The antioxidant property of the obtained compounds was evaluated using 1,1-diphenyl-2-picryl-hydrazyl (DPPH). Results Dammara-12-en-3-one (CB-1), stigmasterol (CB-2) and 3 (2,3,4,5,6-pentahydroxy)-cinnamoyl dammara-12-ene (CB-3) were isolated from the n-hexane fractionate of methanol extract of C. benghalensis. In the study of DPPH radical scavenging activity, IC₅₀ values were predicted to be 790.18, 4186.94 and 2001.16 μg/mL for CB-1, CB-2 and CB-3, respectively, whereas standard ascorbic acid showed IC₅₀ at 1.26 μg/mL. Conclusions Two new dammarane-type triterpene (CB-1 and CB-3) and one phytosterol (CB-2) were identified in C. benghalensis with mild antioxidant property.