A quasi-three-dimensional aerodynamic solver is integrated with a semi-analytical structural weight estimation method inside a multidisciplinary design optimization framework to design and optimize a winglet for a passenger aircraft. The winglet is optimized for minimum drag and minimum structural weight. The Pareto front between those two objective functions is found applying a genetic algorithm. The aircraft minimum take-off weight and the aircraft minimum direct operating cost are used to select the best winglets among those on the Pareto front.