A coolant cooler has a cooling block formed by tubes arranged parallel to one another. The tubes form multiple first flow ducts through which a first fluid can flow. In regions between the tubes multiple second flow ducts are formed through which a second fluid can flow. The coolant cooler includes a first collecting box on which a first fluid inlet is arranged and a second collecting box on which a first fluid outlet is arranged. The first flow ducts are in fluid communication with a first cooling circuit via the first fluid inlet, the first fluid outlet, and the collecting boxes. The first or second collecting box has a second fluid inlet and a second fluid outlet such that the second fluid inlet, the respective collecting box, and the second fluid outlet are in fluid communication with a second cooling circuit.