Abstract: A trailer lock and more specifically to an anti-theft device for use with boat trailers or other trailers having a tongue with a ball socket thereon for engagement with a hitch ball connected to a towing vehicle. The lock includes a hitch ball mounted on top of a vertically adjustable support stand with the ball being received in the ball socket on the trailer tongue. The support stand includes a pivotal lock member which includes a portion closely overlying the upper surface of the ball socket with a padlock assembly securing the lock member in locking position. A lock guard is provided to protect the padlock assembly by preventing easy access to the padlock assembly by bolt cutters or other similar cutting implements.