Six meroterpenoids (compounds 1—6) of chromene class, including three known compounds (1—3), were isolated from Sargassum siliquastrum. The structure of these compounds was established by extensive 2D-NMR experiments such as ¹H gradient double quantum filtered correlation spectroscopy (gDQCOSY), total correlation spectroscopy (TOCSY), nuclear Overhauser effect spectroscopy (NOESY), gradient heteronuclear multiple quantum coherence (gHMQC), and gradient heteronuclear multiple bond correlation (gHMBC), and by comparison with published spectral data. The antioxidant activity of these compounds was evaluated by various antioxidant tests, such as scavenging effects on generation of intracellular reactive oxygen species (ROS), increments of intracellular glutathione (GSH) level, and inhibitory effects on lipid peroxidation in human fibrosarcoma HT 1080 cells. Compounds (1—6) significantly decreased generation of intracellular ROS and inhibited lipid peroxidation while they increased levels of intracellular GSH at a concentration of 5 μg/ml.