A bypass valve for thermally regulating a fluid flow in two directions by opening and closing a bypass fluid flow between the two directions. An example of application of such a bypass valve is for regulating the flow of engine coolant fluid between a vehicle engine and a heat-exchange system for cooling down the engine coolant. The bypass valve includes a housing inside which a first port, a second port, a bypass port and a barrier assembly are arranged. The barrier assembly is movable with respect to the bypass port, so that it can open or close the port depending on fluid temperature. In addition, the barrier assembly is arranged external to the first and second ports, so that it does not interfere with fluid passing through them. The bypass valve therefore provides substantially constant fluid pressure in the first and second ports, avoiding pressure drops associated with conventional bypass valves.