A trailer security system for use with a trailer having a trailer coupling member, such as a trailer coupling channel. The trailer security system may include a security device joined to the trailer coupling member, a locking device to selectively lock the security device to trailer, and security elements attached to trailer. The security device may be a security box formed from walls that define a space for receiving flanges of the trailer coupler channel. The security box may include slots for receiving the trailer coupling member flanges and may be slid onto the trailer coupling member. The security device may include a post extending from the security box for anchoring the trailer to the ground. Access to lock may be restricted by the security elements and/or the configuration of the security box and post. The security system may include other features to increase the difficulty to tamper with or disable the system.