Winemaking by‐products are a natural source of antioxidant components; however, due to their highly perishable and seasonal nature, they may require a prior conservation step before being processed. Natural extracts from fresh and oven‐dried red grape agro‐industrial by‐products were obtained by ultrasound assisted extraction (UAE), using a hydroalcoholic solution as extracting solvent. Extracts were analyzed by HPLC‐DAD‐ESI‐MS, to know the feasibility of winemaking by‐products as natural sources of phenolic compounds, as well as the effect of the oven‐drying treatment on the phenolic composition. Oven‐drying at 45°C caused a significant decrease on the total phenolic content, which implied a reduction of the antioxidant capacity of the extracts. Also, it produced a decrease in total and individual flavan‐3‐ols, stilbenes, and flavonols, being greater in those extracts from stems. Respect to anthocyanins, which were only identified in grape pomace extracts, oven‐drying caused an important decrease, being the peonidin‐3‐O‐glucoside the more thermosensitive compound. Natural extracts from fresh or oven‐dried winemaking by‐products could be used in other food industries as a valuable source of phenolic compounds with antioxidant properties. However, further studies on other drying methods are required for addressing the preservation of phenolic compounds from winery by‐products successfully.