A magnetic field sensor for measuring the rotational speed of a rotating gear component is disclosed. The sensor includes a Hall probe, at least one permanent magnet and housing. The Hall probe is arranged in the influence area of the magnetic field of the permanent magnet. The intent is to produce the magnetic field sensor in a less complex manner. This is achieved by virtue of the fact that the permanent magnet is secured to the housing. One particular advantage of the device is that it is extraordinarily robust and is only slightly sensitive to vibrations.