Warning device for a trailer hitch and trailer hitch equipped with a warning device

The warning device (14) has at least one sensor (36) to report the locking position of the locking device (13). The sensor has an electricity supply (41) or is connected to an external one. Visual and acoustic warning is given if the sensor detects that the locking device is not in the locking position when the headpiece (12) is in the coupling holder (11).