Oxidative stress, an imbalance between reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) and the antioxidants present in the organism, has a significant role in the pathogenesis of various illnesses. Moreover, this process contributes to cellular damage, affecting DNA, proteins, and lipids, inducing cellular aging and age-related diseases such as cancer, cardiovascular diseases, and neurodegenerative disorders [1]. Brown macroalgae have been demonstrated to be significant potential ROS and RNS scavengers, primarily due to their rich antioxidant properties. Various studies have highlighted the ability of brown algae extracts to neutralize ROS and RNS, including superoxide (O₂^●-) and hydroxyl (^●OH) radicals, hydrogen peroxide (H₂O₂), and nitric oxide (^●NO), which are known to contribute to oxidative stress. This antioxidant capability of these extracts is attributed to the presence of bioactive compounds such as polyphenols, specifically phlorotannins, among other secondary metabolites found in these marine species. In this work, six brown algae species, i.e., Himanthalia elongata, Undaria pinnatifida, Sargassum muticum, Bifurcaria bifurcata, Fucus spiralis, and Ascophylum nodosum, were studied, aiming to assess their ability to promote ROS and RNS scavenging. The bioactive compounds were extracted by microwave-assisted extraction [2–5], and the scavenging activity was tested by in vitro spectrophotometric methods [6]. The results showed that concerning the ^●NO scavenging activity, the EC₅₀ varied from 74 µg/mL to 2000 µg/mL for Fucus spiralis and Himanthalia elongata, respectively. The EC₅₀ regarding the O₂^●- scavenging activity ranged from 16 µg/mL (Sargassum muticum) to 279 µg/mL (Undaria pinnatifida), while for H₂O₂ scavenging, the best result was achieved by Fucus spiralis (EC50 =39 µg/mL). The radical ^●OH was the reactive species that required the highest quantity of algae extract to achieve the 50% depletion, averaging 1240 µg/mL. These results highlight the activity of the algae extracts as antioxidants, showing their potential to reduce oxidative stress.