Marine cyanobacteria are known to produce structurally diverse bioactive specialized metabolites during bloom occurrence. These ecologically active allelochemicals confer chemical defense for the microalgae from competing microbes and herbivores. From a collection of a marine cyanobacterium, cf.Lyngbyasp., a small quantity of a new cyclopropane-containing molecule, benderadiene (2), and lyngbyoic acid (1) were purified and characterized using spectroscopic methods. Using live reporter quorum-sensing (QS) inhibitory assays, based onP. aeruginosaPAO1lasB-gfpandrhlA-gfpstrains, both compounds were found to inhibit QS-regulated gene expression in a dose-dependent manner. In addition to lyngbyoic acid being more active in the PAO1lasB-gfpbiosensor strain (IC50of 20.4 µM), it displayed anti-biofilm activity when incubated with wild-typeP. aeruginosa. The discovery of lyngbyoic acid in relatively high amounts provided insights into its ecological significance as a defensive allelochemical in targeting competing microbes through interference with their QS systems and starting material to produce other related analogs. Similar strategies could be adopted by other marine cyanobacterial strains where the high production of other lipid acids has been reported. Preliminary evidence is provided from the virtual molecular docking of these cyanobacterial free acids at the ligand-binding site of theP. aeruginosaLasR transcriptional protein.