Abstract: A gooseneck trailer coupler lock which disables normal coupler operation by restricting movement of the lower latching plate of the coupler when the coupler is placed in the fully closed position. The locking device is a single piece of hardened metallic material which inserts into a gap formed between the upper coupler plate and a band attached to the lower latching coupler plate. The locking device is secured in place with a block type lock or other theft resistant lock. When the locking device is installed, the coupler opening is secured in a position which will not accept a typical towing ball without first removing the locking device.