A keyway protector having housing and the slider. The housing hugs the crossbar of a U-lock and secured by screws. The slider fits into a shallow depression in the inner surface of the housing and moves between open and closed positions. In the open position, an aperture in the housing, a hole in the slider, and the keyway are aligned to allow access to the keyway. In the closed position, the slider hole is not aligned, denying access to the keyway. A knob outside the housing is used to move the slider. Protrusions in opposed pivoting arms on the slider snap into opposed notches in the depression walls to hold the slider in the open or closed position.