Marine macroalgae are gaining importance in the diet due to their nutritional composition and potential health benefits. Many of these beneficial properties are motivated by the presence of antioxidant compounds, responsible for the improvement of product preservation, and their therapeutic effects in the prevention of various diseases. Among all the marketed algae, one of the most consumed is Himanthalia elongata, known as sea spaghetti. However, despite such high consumption, there are only few studies focused on its chemical characterization. Therefore, different studies are required to optimize the extraction and characterization of its bioactive compounds, specially polyphenols with antioxidant activity. This study is aimed at optimizing of the extraction of antioxidants from H. elongata by ultrasound-assisted extraction (UAE). This system was chosen as it is an emerging green technique with which good results are achieved in plant matrices, as supported by several studies. The process was optimized by response surface methodology (RSM) using a five-level central composite design, combining the independent variables of processing time (t, 15-45 min), power (P, 0-100%) and solvent (S, 0-100 % ethanol, v/v) [1]. Specifically, RSM was performed to optimize different responses associated with polyphenol production and antioxidant activity determination: total phenolic content, total flavonoids, antioxidant capacity, in terms of sequestering capacity of the diphenyl-2-picryl-hydrazyl radical (DPPH), Trolox equivalent antioxidant capacity (TEAC) and β-carotene discoloration method (BC), and extraction yield. The theoretical models were fitted to the experimental data, statistically validated, and used in the prediction and optimization steps. The best yields of fucoxanthin, a compound with antioxidant activity typical of brown algae, were obtained at 44.9 min, 418.9 W and 86.8% ethanol. The analysis presented provides important data that allows the comparison between different extraction conditions, in terms of efficiency, and consequent related decision making. Overall, it can be concluded that ultrasounds assisted extraction can be an efficient and green technique to revalorize algae of common uses as Himanthalia elongata into potent antioxidant phenolic for their further application in food and nutraceutical industries.