A bypass valve for blocking oil flow through a radiator from a transmission with low oil temperatures includes an oil supply from the radiator, an oil supply, via a bypass line, an oil outlet and a valve unit arranged inside the valve housing. The valve unit includes a compression spring and a heat sensitive element, which move depending on the oil temperature such that when the temperature falls below a threshold value, the oil supply from the radiator is closed and the supply, via a bypass line, is opened. When the oil temperature rises above the threshold value, the oil supply from the radiator is opened and the supply, via the bypass line, is closed. The valve unit can be axially moved against the force of the compression spring, when a predefined oil pressure is reached, thus opening the supply, via the bypass line.