Humidifier for a fuel cell system for humidifying a dry cathode supply air (K-ZL) with a humid cathode waste air (K-AL), - wherein the humidifier has a housing, a cuboid membrane block consisting of a plurality of membranes stacked at a distance from one another transversely to a longitudinal direction of the membrane block, and a sealing arrangement, - wherein the membrane block has two end surfaces (6a, 6b) which are closed in an airtight manner and lie opposite one another and four flow-through surfaces (7a, 7b, 7c, 7d) forming the circumference of the membrane block, - wherein the membrane block is accommodated in the housing in such a way that an air chamber (8a, 8b, 8c, 8d) is formed in each case between the respective flow-through surface (7a, 7b, 7c, 7d) of the membrane block and an opposite region of the housing for supplying or discharging the cathode waste air (K-AL) or the cathode supply air (K-ZL) to/from the membrane block, - wherein the respective air chambers (8a, 8b, 8c, 8d) inside the housing are separated from one another in an airtight manner by means of the sealing arrangement, characterised in that - the sealing arrangement is formed from three or four annular seals (5a, 5b, 5c, 5d) which are separate from one another, wherein the respective annular seal (5a, 5b, 5c, 5d) is associated in each case with one of the flow-through surfaces (7a, 7b, 7c, 7d) of the membrane block, and - in that the respective annular seal (5a, 5b, 5c, 5d) runs around the edge of the respective associated flow-through surface (7a, 7b, 7c, 7d) and is arranged in a sealing manner between the respective flow-through surface (7a, 7b, 7c, 7d) and the housing. - wherein the humidifier has a housing, a cuboid membrane block consisting of a plurality of membranes stacked at a distance from one another transversely to a longitudinal direction of the membrane block, and a sealing arrangement, - wherein the membrane block has two end surfaces (6a, 6b) which are closed in an airtight manner and lie opposite one another and four flow-through surfaces (7a, 7b, 7c, 7d) forming the circumference of the membrane block, - wherein the membrane block is accommodated in the housing in such a way that an air chamber (8a, 8b, 8c, 8d) is formed in each case between the respective flow-through surface (7a, 7b, 7c, 7d) of the membrane block and an opposite region of the housing for supplying or discharging the cathode waste air (K-AL) or the cathode supply air (K-ZL) to/from the membrane block, - wherein the respective air chambers (8a, 8b, 8c, 8d) inside the housing are separated from one another in an airtight manner by means of the sealing arrangement, characterised in that - the sealing arrangement is formed from three or four annular seals (5a, 5b, 5c, 5d) which are separate from one another, wherein the respective annular seal (5a, 5b, 5c, 5d) is associated in each case with one of the flow-through surfaces (7a, 7b, 7c, 7d) of the membrane block, and - in that the respective annular seal (5a, 5b, 5c, 5d) runs around the edge of the respective associated flow-through surface (7a, 7b, 7c, 7d) and is arranged in a sealing manner between the respective flow-through surface (7a, 7b, 7c, 7d) and the housing.