Abstract: A locking device for locking an outboard stern drive mechanism against removal from the stern of a boat, the locking device including a U-shaped cover member adapted to cover the stern drive mechanism. Support brackets are secured outboard of the stern and flanking the stern drive mechanism. The U-shaped cover member is pivotally connected to the brackets so that the cover member can be moved between a raised position providing access to the stern drive mechanism and a lowered position for covering the stern drive mechanism to prevent theft thereof. A locking member securely locks the cover member in its lowered position. The locking member includes a locking bar having a finger for extending into an opening in the stern drive mechanism so as to prevent the steering of the boat and operation of the stern drive mechanism.