The chemical composition of a new bioactive extract from immortelle sandy (Helichrysum arenarium (L.) Moench.) was investigated. Naringin, its soluble aggregate, prunin, quercetin, apigenin, naringenin, apigenin 5-O-glucoside, and isosalipurposide were found among the flavonoids. The tested immortelle sandy extract was shown to contain known dimers, trimers, or more complex aggregates. The compounds had similar spectral but substantially different chromatographic characteristics, which can be used for their identification based on the corresponding data on flavonoids glycosylated at different positions and/or carbohydrates of various complexities, which are also characterized by similar absorption spectra and different retention times. It was found by molecular absorption spectroscopy that the immortelle extract contained 73.48 mg of flavonoids relative to rutin or 17.94 mg relative to quercetin per 1 g of dry extract weight, which corresponded to 20.99 and 5.13%, respectively. The immortelle extract obtained by the method proposed displayed antitumor activity against the transplanted sarcoma 45 and had a beneficial effect on animals in general.