A device (1) for frothing a liquid comprises a tube-shaped housing (10). An inner surface (13) of the housing (10) has a curved periphery. At least one inlet (20) is arranged in a lateral area (12) of the housing (10) for letting in the liquid and a frothing gas, and may have a tangential orientation with respect to the curved periphery of the inner surface (13) of the housing (10). End faces (14a, 14b) of the housing (10) are open and function as outlets of the device (1), wherein it is preferred for a longitudinal axis (15) of the housing (10) to extend in a horizontal direction. Rings (21a, 21b) may be applied inside the housing (10) for keeping a rotating flow inside the housing (10) during a period of time which is sufficiently long for a frothing process to take place in an effective manner.