A diverter damper for controlling a gas flow in a gas duct of large cross section. The diverter damper includes a housing having an inlet and two outlets, a pivotable flap which in a first extreme position closes a first outlet and in a second extreme position closes a second outlet. The damper further includes a drive shaft connected to the pivotable flap, wherein the drive shaft extends at least partly between two opposite housing walls and through at least one of the two opposite housing walls, and at least one actuator mechanism that is located outside the housing near or against the at least one housing wall through which the drive shaft extends. The actuator mechanism includes at least one cylinder piston unit connected to the drive shaft for pivoting the flap into one of the extreme positions or into a position between the extreme positions.