The purpose of this paper is to present a tuned mass damper which simultaneously reduces the vertical and torsional buffeting responses of long-span bridges. The proposed damper has two frequencies, which are tuned to the frequencies of the first flexural and torsional structural modes, to suppress the resonant effects. The aerodynamic coupling is taken into account for the formulation of the bridge-damper system, and this model is thus applicable to those bridges with significant mode coupling. The buffeting response reduction and the increase of the critical velocity are investigated through a parametric analysis. Based on this parametric study, the design procedures of the tuned mass damper for the wind-excited bridges are proposed. The results show that the proposed tuned mass damper is at least as effective as the usual tuned mass damper for suppressing buffeting response; furthermore it appreciably increases the stability of the bridge with either a single-degree-of-freedom flutter or a coupled flutter.