In this study, the bioactive properties and phenolic constituents of different grape seeds grown in Turkey were investigated. Total phenolic and flavonoid contents of grape seeds varied between 1,120.83 mgGAE/100 g (Tarsus beyazı) and 3,485.24 mgGAE/100 g (Büzgülü) to 10,087.13 mg/100 g (Topacık) and 47,013.06 mg/100 g (Ekşikara),respectively. Antioxidant activity values of grape seeds were determined between 79.22%(Kızıl Üzüm-Sık Dimrit) and 90.24% (Büzgülü-Konya). In general, the total phenolic contents of Büzgülü (Gülnar-Mersin), Miski, İri Aküzüm, Büzgülü (Konya), İspitiren, Göküzüm, İrikara, Dimrit and Muftalma grape seeds were above 2,000 mgGAE/100 g. While (+)-catechin values of seeds are determined between 30.70 (Devegözü-Black) and 410.42mg/100 g (İspitiren),1,2-dihydroxybenzene amounts of extracts were found between 31.43 (Devegözü-Black) and 1,211.00 mg/100 g (Hönüsü). While apigenin-7-glucoside values of grape seed extracts vary between 42.22 (Dimrit) and 293.58 mg/100 g (Kara Dimrit), quercetin contents of seeds changed between 91.79 (Dimrit) and 466.44 mg/100 g (Dimrit-Siyah). A large variation of amounts of phenolic constituents depending on grape seed varieties was observed. Practical applications: Grapes are one of the fruits with the highest phenolic compound content. Therefore, grape seed obtained as a by-product during the production of boiled juice, wine and juice is a good source of polyphenols. In the grape processing industries, these by-products emerge as potential waste, which is a problem for the environment. Grape seeds, which are obtained in large amounts in the fruit juice, jam, marmalade, boiled fruit juice industries as grape by-products, are a good source of antioxidants and phenolic components.