This invention is a new and distinct grapevine variety denominated ‘IFG Thirty-one’. The new grapevine ‘IFG Thirty-one’ is characterized by producing small seedless, purple-black berries having a narrow ellipsoid shape with a unique strong fruity labrusca flavor reminiscent of artificial grape flavor. Fruits normally ripen early to mid-season about late July to early August near Delano Calif.