Abstract: A locking device for a trailer hitch of the type which is used to connect the rear end of a pickup truck with the forward end of a trailer, the trailer having an adapter arm connected with the forward end of the trailer and being provided with a socket for engaging a ball on the rear end of the pickup truck, the ball having a threaded portion which projects downwardly through a support plate and having a nut secured to the lower threaded portion of the ball, a locking plate pivotally connected to the adapter arm at a location between the ends of the arm, the locking plate being provided with a first hole whose diameter is larger than the outer diameter of the nut such that, when the locking plate is pivoted towards the support plate, the lower end of the ball and the nut will be received in the first hole and the locking plate can be adjacent the underside of the support plate, a locking chain having a first end and a second end, the first end of the locking plate being connected to the locking plate intermed