Trailer coupling lock for blocking operation of a release handle of a trailer coupling, - wherein the trailer coupling lock comprises an adjustable fastening means for fastening the trailer coupling lock on trailer couplings of different sizes, and wherein the trailer coupling lock further comprises a blocking portion to which a blocking device can be connected and be locked, in a way that it can be unlocked, in order to together block operation of the release handle in a locked position, - wherein the fastening means comprises a first and a second support means located opposite at a distance from each other and mutually fixed, - wherein at least the first support means comprises a portion with a thread and further a sleeve with a first thread which can be screwed off and on the thread of the portion such that the sleeve can be adjusted to clamp a trailer coupling between the sleeve and the second support means in a fixed position, - wherein inside the sleeve is a second thread located, the pitch of said second thread is different from the pitch of the first thread of the sleeve, - wherein the fastening means further comprises a fixing screw with a screw head and a thread, which thread fits with the second thread of the sleeve, - wherein the second support means has a recess in which the head of the fixing screw can be completely countersunk such that the head is closely enclosed by the second support means, - wherein the fixing screw, in the fixed position, is introduced through the second support means, through a trailer coupling fixed between the sleeve and the second support means, and further is screwed with its thread into the second thread inside the sleeve, and lies with its head in the recess of the second support means and against the second support means such that an axial displacement of the fixing screw in a direction towards the sleeve is prevented, and wherein the head of the fixing screw in the fixed position is sealed to prevent an unscrewing of the fixing screw with a screwing tool, whereby, in the fixed position, a rotation attempt on the sleeve, in a direction that wants to carry the sleeve away from the fixed trailer coupling, causes a locking of the sleeve by means of the two different threads (10, 11) of the sleeve with different pitches such that the sleeve is prevented from being unscrewed from the trailer coupling in the fixed position. - wherein the trailer coupling lock comprises an adjustable fastening means for fastening the trailer coupling lock on trailer couplings of different sizes, and wherein the trailer coupling lock further comprises a blocking portion to which a blocking device can be connected and be locked, in a way that it can be unlocked, in order to together block operation of the release handle in a locked position, - wherein the fastening means comprises a first and a second support means located opposite at a distance from each other and mutually fixed, - wherein at least the first support means comprises a portion with a thread and further a sleeve with a first thread which can be screwed off and on the thread of the portion such that the sleeve can be adjusted to clamp a trailer coupling between the sleeve and the second support means in a fixed position, - wherein inside the sleeve is a second thread located, the pitch of said second thread is different from the pitch of the first thread of the sleeve, - wherein the fastening means further comprises a fixing screw with a screw head and a thread, which thread fits with the second thread of the sleeve, - wherein the second support means has a recess in which the head of the fixing screw can be completely countersunk such that the head is closely enclosed by the second support means, - wherein the fixing screw, in the fixed position, is introduced through the second support means, through a trailer coupling fixed between the sleeve and the second support means, and further is screwed with its thread into the second thread inside the sleeve, and lies with its head in the recess of the second support means and against the second support means such that an axial displacement of the fixing screw in a direction towards the sleeve is prevented, and wherein the head of the fixing screw in the fixed position is sealed to prevent an unscrewing of the fixing screw with a screwing tool, whereby, in the fixed position, a rotation attempt on the sleeve, in a direction that wants to carry the sleeve away from the fixed trailer coupling, causes a locking of the sleeve by means of the two different threads (10, 11) of the sleeve with different pitches such that the sleeve is prevented from being unscrewed from the trailer coupling in the fixed position.