The filter cartridge for the water tank of a coffee machine comprises a housing having on both sides a plurality of slot-shaped openings constituting two inlets. For softening the water, the housing of the filter cartridge contains a ion exchange resin. The bottom of the housing comprises a central outlet. The interior of the housing is provided with a plurality of baffle members increasing the length of the flow path of the water between inlets and outlet. The housing comprises an upper portion and a lower portion, sealingly fixed to each other. The baffle members extend essentially in vertical direction and are alternately fixed to the upper and to the lower housing portion. Water passages are provided between the upper housing portion and the baffle members fixed to the lower housing portion, as well as between the lower housing portion and the baffle members fixed to the upper housing portion. The filter cartridge of the invention requires not much space, but offers high efficiency resulting from the long flowing path of the water through the ion exchange resin between inlets and outlet.