The invention relates to a temperature sensor (5) for measuring a temperature of a battery system (1) for storing electrical energy and also for supplying an electric motor of a motor vehicle with electrical energy. The temperature sensor (5) has a sensor head (6) and connection wires (7) electrically coupled to the sensor head. At least one of the connection wires (7) is at least partially formed as a spring element (8) or forms at least part of a spring element (8), wherein the spring element (8) is formed as a helical spring or as twisted. The invention further relates to a battery system (1) with at least one temperature sensor (5) for measuring a temperature of at least one battery cell (3), wherein the temperature sensor (5) has at least one spring element (8), which is mechanically attached to a sensor head (6) of the temperature sensor (5) and is fastened to the battery management system (4) in such a way that the sensor head (6) is pressed by the spring element (8) onto a measuring point (9) on a battery unit (2) of the battery system (1). The invention also relates to a method for fitting such a battery system (1).