The present invention provides a locking device for a hinged, body and cap trailer coupler to prevent unauthorized use of an unattended and unattached trailer. The device consists of a cylindrical bar member joined to an elongated tongue member. The elongated tongue member is sized for insertion between the body and cap portion of a trailer coupler in the open position such that the cap portion is forced against the sliding collar of the trailer coupler, preventing the trailer coupler from closing on the ball of a tow vehicle. The elongated tongue member provides a slotted hole portion to receive the shackle of a padlock to lock the system in place.