One new dihydrochalcone diglycoside has been isolated from the EtOAc fraction of the Egyptian seagrass Thalassodendrin ciliatum (Forsk.) den Hartog, and was identified as 6'-O-rhamnosyl-(1‴ → 6″)-glucopyranosyl asebogenin for which a trivial name Thalassodendrone was established. Furthermore, five known phenolics were isolated and identified as asebotin, quercetin 3,7-diglucoside, protocatechuic acid, ferulic acid and p-hydroxybenzoic acid. The structures of all the isolated compounds were established based on 1D and 2D NMR spectroscopy and high-resolution-mass spectrometer. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were obtained using a JEOL JMS-T100TD spectrometer (JEOL Ltd., Tokyo, Japan). The anti-influenza A virus activity of the isolated new compound and asebotin was evaluated, and the obtained results revealed that the inhibition dose concentration of asebotin was more than that of Thalassodendrone with IC50 = 2.00 and 1.96 μg/mL, respectively, and with cytotoxic concentration (CC50) of 3.36 and 3.14 μg/mL, respectively.