An electrochemical sensor (1, 101) has a sensor element (15, 115) with a measuring surface (8, 108) that faces a measuring medium (5) during use. The sensor element has a planar measuring element (2, 102). A sensor shaft (4, 104) has an aperture (13, 113) with a bezel (11, 111) at an end which, during use, faces the measuring medium. The sensor element is installed in the area of the aperture. The electrochemical sensor also has an annular sealing element (9, 109), which is arranged between the sensor element and the bezel. An insulator element (10, 110) is firmly and inseparably connected to the measuring element, exposing or recessing the measuring surface. Thus, the sealing element, which protects the electrochemical sensor against the ingress of measuring medium, is arranged sealingly between the insulator element and the bezel.