This study investigated the allelopathic activity of sixteen seaweed extracts using lettuce seed germination bioassay. The results of the preliminary investigation suggested that the methanol extracts of seven seaweeds demonstrated significant properties inhibitory to seed germination. In addition, chemical investigation of an active extract of the alleopathic Harvey led to the isolation of four known compounds as well as a new brominated nonaromatic isolaurene type sesquiterpene. The structural elucidation of pure compounds was carried out using spectroscopic methods and by comparison with literature. As seaweeds are highly abundant around Sri Lanka, they could be developed as an ecofriendly natural herbicide in their crude form.