A new and distinct cultivar of Dahlia plant named `Datretten` characterized by its ray floret color (when opening, upper side): base: red, RHS 44B and apex: white, RHS N155B; capitulum (composite flower head): 9.5 to 10.5 cm in diameter, with a large number of ray florets (about 100 to 140); mature leaves about 8 to 12 cm in length, mostly compound, with 1-5 leaflets, and immature leaves about 4 to 6 cm in length, compound, with 1-5 leaflets; and vigorous plant vigor, growing upright with slight spread to form cylindrical-shaped plant.