The author introduces a new method to determine crustal stress changes from tectonomagnetic field variations. The principal part of the new approach is a structural model of a geological formation with stress dependent magnetic rock properties, which acts as a natural geomagnetic stress sensor. The new technique was applied to data obtained from the investigation area of the German-Turkish Earthquake Prediction Research Project at the western end of the North-Anatolian Fault Zone. Here, the differential magnetic total intensity field is being measured biannually since Mai 1985. The author compares the stress changes determined from tectonomagnetic field variations with the results obtained from other experiments and demonstrates that the new method could be significant for monitoring crustal stress changes.