A retractable trailer cover adapted for mounting to an existing boat trailer and a method for retrofitting the trailer cover to the boat trailer. The trailer cover includes multiple ribs extending from one side to the other, a flexible cover material, two elongated tracks, and multiple wheels received within the tracks. The ribs are affixed to the wheels so as to be slidably mounted to the tracks. Mounting brackets can be spaced along each of the trailer cover sides, and the tracks can be affixed to the mounting brackets. To mount the trailer cover to the boat trailer, the mounting brackets can be attached to a frame of the boat trailer. The trailer cover can selectively be expanded and retracted to cover and uncover an associated boat and boat trailer and may be used with new or used trailers.