Abstract: This invention relates to a locking device, having a U-shaped cross-section, for use with bi-fold doors. The device is designed to be slidably mounted on the top edge of one panel of a bi-fold door in the open position. When the door is closed, the device is slid over the top edge of both panels, and a bolt portion is inserted in the space between the two panels. To open the door, the locking portion is simply removed from said space and the locking device is moved to the top edge of just one of the panels.