The grape skin extract obtained from Alicante Bouschet (Vitis vinifera L.) presented anthocyanins (396 mg/L), phenolic acids (76 mg/L) and flavanols (63 mg/L). After in vitro digestion and colonic fermentation there was a significant reduction of anthocyanins and flavanols but there was an increase of phenolic acids, mainly gallic acid and also a concomitant increase of antioxidant capacity. There was no difference for the microorganisms after 24 h of colonic fermentation but there was a significant increase of acetic acid (p < 0.05). The bioaccessibility of phenolic acids was significantly improved (>100%) and grape skin extract favored the metabolic activity of the microbiota.