A new and distinct cultivar of Dahlia plant named `DA12` characterized by its ray floret color (when opening, upper side): Red, RHS 44A.; capitulum (composite flower head): 9.5 to 11.0 cm in diameter, with a large number of ray florets (about 110 to 150); mature leaves are about 6 to 15 cm in length, mostly compound, with 1-5 leaflets and immature leaves are about 4 to 10 cm in length, compound, with 1-5 leaflets; and vigorous plant vigor, growing upright to outward to form cylindrical-shaped plant.