**Abstract**

It is well established in the scientific community that agro-food wastes represent economic advantages and contribute to circular economy. For instance, wine industries of Região Demarcada do Douro involve the production of large quantities of by-products, such as stem, pomace, trimmed vine shoots, or wine lees, presenting a remarkable valuable composition in phytochemicals with putative health-promoting qualities. Nevertheless, the bioactive compounds obtained from these natural sources depends on the extraction process employed. In order to reduce production costs and optimize processes, new technologies—such as ultrasound-assisted extraction (UAE)—have been employed to decrease energy consumption and increase the product or process safety/control and quality. This work aims to characterize the phenolic compounds extracted from winery byproducts (WBPs), namely grape stems, grape pomace, and wine lees of two grape (*Vitis vinifera* L.) varieties (Sousão and Tinta Barroca) from the same geographical site, as well as the antioxidant capacity. Wine lees and grape stems presented the highest concentration of phenolic compounds and the highest antioxidant capacity for Tinta Barroca variety, while grape pomace presented the highest values of these parameters for Sousão variety, demonstrating the high influence of the variety studied. Furthermore, wine lees revealed to be the winery by-product with the lowest antioxidant capacity and content of phenolics. These by-products revealed to be a rich source of phenolic compounds with high antioxidant capacities reveling to be of interest for pharmaceutical and cosmetic industries.

**Keywords:** Winery by-products, Ultrasound-assisted extraction, Bioactive compounds, Antioxidant capacity, Valorization
