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Background/Objectives: Syringa vulgaris L. (common lilac) is one of the most popular ornamental plant species. Through the ages, many parts of S. vulgaris, including fruits, flowers, leaves, and branches, have been used in folk medicine due to their beneficial biological activity. Lilac flowers are the basis of many supplements available on the market. Moreover, its petals and flowers are edible and are an aromatic ingredient in preserves and desserts. However, the data about the antioxidant properties of various parts of S. vulgaris is limited only to the in vitro antioxidant capacity of the extracts—so far, the effect of S. vulgaris flower extract on the parameters of oxidative stress in biological materials, including plasma, has not been demonstrated. Therefore, the aim of our study was to investigate the protective effects of the extract from S. vulgaris L. flowers against oxidative stress in human plasma, and its influence on the coagulation process in vitro. Methods: We measured the levels of three parameters of oxidative stress in human plasma treated with H2O2/Fe2+ (the donor of hydroxyl radicals): lipid peroxidation (based on the level of thiobarbituric acid reactive substances (TBARS)), protein carbonylation, and thiol oxidation. Ascorbic acid (vitamin C) was used as a reference antioxidant. In addition, we studied the effect of the extract on three coagulation parameters of human plasma-activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT). We also compared the biological properties of the extract from S. vulgaris flowers with the properties of a phenolic extract from Taraxacum officinalis (dandelion) flowers, as they have proven antioxidant activity in both in vitro and in vivo models and can modulate hemostasis in vitro. Results: Our UHPLC-HRMS analyses of S. vulgaris extract led to a tentative identification of 50 compounds, mainly phenolics and secoiridoids. For the first time, the present study demonstrated that the extract from S. vulgaris flowers (at the concentrations of 1–50 µg/mL) significantly reduced plasma lipid peroxidation and protein carbonylation induced by H2O2/Fe2+. Moreover, the concentrations of 1–25 µg/mL significantly reduced the oxidation of thiol groups in plasma treated with H2O2/Fe2+. The anticoagulant tests also demonstrated that S. vulgaris flowers extract, at physiologically relevant concentrations (1–50 µg/mL), did not affect blood clotting times in vitro, suggesting that it is hemostatically safe. Conclusions: Despite the differences in composition, the extracts from lilac flowers and dandelion flowers exhibited similar protective effects against oxidative damage to human plasma components. However, the extract from S. vulgaris flowers had a stronger inhibitory effect on lipid peroxidation than the extract from dandelion flowers.