A superheat sensor (1) for sensing superheat of a fluid flowing in a flow channel (3) is disclosed. The sensor (1) comprises a flexible wall defining an interface between an inner cavity (5) having a charge fluid (6) arranged therein and the flow channel (3). The flexible wall is arranged in the flow channel (3) in thermal contact with the fluid flowing therein, and the flexible wall is adapted to conduct heat between the flow channel (3) and the inner cavity (5). Thereby the temperature of the charge fluid (6) adapts to the temperature of the fluid flowing in the flow channel (3), and the pressure in the inner cavity (5) is determined by this temperature. A first wall part (7, 14) and a second wall part (9, 16) are arranged at a variable distance from each other, said distance being defined by a differential pressure between the pressure of the charge fluid (6) and the pressure of the fluid flowing in the flow channel (3), i.e. depending on the pressure and the temperature of the fluid flowing in the flow channel (3), and thereby the superheat of the fluid. A distance sensor, e.g. comprising a permanent magnet (8) and a Hall sensor (10), measures the distance between the wall parts, and the superheat is calculated from the measured distance. The sensor (1) is suitable for use in a refrigeration system. The sensor (1) is mechanically simple and capable of determining the superheat by measuring only one parameter.