Knema angustifolia Warb (Myristicaceae) is used in traditional Thai medicine as a whole body tonic agent. The current study sought to evaluate the antioxidant, antibacterial and cytotoxic activities of extracts from this herb and to screen their chemical constituents. Antioxidant, antibacterial and cytotoxic activities were determined by the DPPH scavenging method, the disk diffusion method and the colorimetric method, respectively. Phytochemical screening was performed using standard procedures. The results showed that the ethanol extract had high antioxidant activity with an EC ₅₀ value of 13.90 ± 1.35 μg/ml. The ethanol and the dichloromethane extracts were active against Staphylococcus aureus. The ethanol extract showed a strong cytotoxic activity against the lung cancer cell line with an IC50 value of 4.55 ± 4.60 μg /ml. Phytochemical screening of the extracts showed the presence of condensed tannins, phenolic compounds and triterpenes. In conclusion, the apparent antioxidant and cytotoxic activities of K. angustifolia suggest its potential usefulness in the prevention of cancer and other diseases.