Stem byproducts from 10 different grape (Vitis vinifera L.) varieties were evaluated in terms of their total phenolic and total proanthocyanidin contents, flavan-3-ol and proanthocyanidin profiles, and antioxidant capacity measured by ABTS, CUPRAC, FRAP, and ORAC assays, with a view to the recovery of their natural bioactive compounds. Stems from Callet, Syrah, Premsal Blanc, Parellada, and Manto Negro varieties yielded the highest total phenolic and total proanthocyanidin contents and showed the greatest antioxidant capacities, whereas Chardonnay and Merlot stems presented the lowest values. Varieties differed significantly (p<0.05) with regard to both the phenolic composition and antioxidant capacity of their stems. However, no significant differences (p>0.05) were observed when stems from red and white varieties were considered separately. For the 10 grape varieties investigated, this is the first study presenting a detailed description of their stem flavan-3-ol composition determined by HPLC-UV-fluo. All of the analyses confirmed the stem byproducts as a potential polyphenol-rich source, especially promising in the case of the Callet variety.