This flat tube heat exchanger encompasses a closed housing, in which two tube sheets and a tube bundle, which is arranged between the tube sheets and which is supported by the tube sheets is arranged. The tube bundle comprises at least some flat tubes, which extend in longitudinal direction of the tube bundle. At their ends, the flat tubes are round and are flat in a central section. The ends of the flat tubes, which have a round cross section, can be circular or can encompass a different round shape.