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Dodder (<i>Cuscuta</i> spp.) is a parasitic weed damaging many plants and agricultural production. The native obligate parasite <i>Cuscuta japonica</i> Choisy (Japanese dodder) parasitizes <i>Dimocarpus longans</i> Lour., <i>Ficus septica</i> Burm. F., <i>Ficus microcarpa</i> L.f., <i>Mikania micrantha</i> H.B.K. and <i>Melia azedarach</i> Linn, respectively. Five Japanese dodders growing on different plants exhibit slightly different metabolites and amounts which present different pharmacological effects. Among these plants, a significant antiviral activity against influenza A virus (IAV) was found in Japanese dodder parasitizing on <i>D. longans</i> Lour. (CL). To further explore methanol extract components in Japanese dodder (CL), four undescribed aromatic glycosides, cuscutasides A–D (compounds <b>1</b>–<b>4</b>) were isolated, together with twenty-six known compounds <b>5</b>–<b>30</b>. The chemical structures of <b>1</b>–<b>4</b> were elucidated using a combination of spectroscopic techniques. The eighteen isolated compounds were evaluated for antiviral activity against IAV activity. Among them, 1-monopalmitin (<b>29</b>) displayed potent activity against influenza A virus (A/WSN/1933(H1N1)) with EC₅₀ 2.28 ± 0.04 μM and without noteworthy cytotoxicity in MDCK cells. The interrupt step of <b>29</b> on the IAV life cycle was determined. These data provide invaluable information for new applications for this otherwise harmful weed.