Hakea sericea has been introduced to Portugal for ornamental purposes. The phytochemical composition and the antioxidant, antibacterial, antibiofilm and cytotoxic properties of this shrub species have been previously reported. The present work describes the bioassay-guided fractionation of the crude methanolic extract of H. sericea fruits and the isolation of 9-(3,5-dihydroxy-4-methylphenyl)nona-3(Z)-enoic acid. The structure of this new compound was established by one- and two-dimensional NMR and IR spectroscopy, and high-resolution mass spectrometry. The antibacterial properties of the new alkenylresorcinolwere studied by determining its MIC values against several strains of Gram-positive and Gram-negative bacteria using the resazurin microtiter assay. The new alkenylresorcinol inhibited the growth of Enterococcus faecalis, Listeria monocytogenes and Bacillus cereus with MIC values of 0.31, 0.02 and 0.16 mg/mL, respectively. Good MIC values were obtained against Staphylococcus aureus strains (0.005 - 0.16 mg/mL), including the clinical isolates (SA 01/10, SA 02/10 and SA 03/10) and MRSA strains.