A bypass valve for a cooler connected downstream from a hydraulic aggregate. The bypass valve includes a housing with ports for connection to the hydraulic aggregate and the cooler. A closure element is located in the housing for closing and opening a bypass. A spring pushes the closure element into a closing position, and thermo-sensitive means causes the closure element to open if the temperature of the hydraulic fluid is below a threshold temperature. The thermo-sensitive means are formed by the valve spring, which is made of a memory-metal alloy with a transition point close to the threshold temperature such that below the threshold temperature it no longer exerts a force and allows the closure element to open.