A mounting bracket lock may be configured to secure the mounting bracket of a window treatment housing to a rail of the housing. The mounting bracket lock may be slidably attachable to the rail and, when in a locked position, may prevent the mounting bracket from detaching from the rail. The mounting bracket lock may include a body that is mountable to the rail. The body may include a first portion that slidably attaches to the rail, and a second portion that receives a portion of the rail and a portion of the mounting bracket, thereby securing the mounting bracket in a locked position relative to the rail. The body of the mounting bracket lock may define one or more projections that extend therefrom, and that engage in a friction fit between corresponding surfaces of the mounting bracket when the mounting bracket lock is in the locked position.