A cap for a writing instrument includes a cap body having a peripheral wall formed with an elongated mounting recess, a retaining cover removably mounted in the mounting recess of the cap body, and a magnet mounted between the cap body and the retaining cover. Thus, the cap and the writing instrument are attached to the surface of a whiteboard or other metal by the magnetic force of the magnet, so that a user can use the writing instrument easily and conveniently, thereby facilitating the user using the writing instrument, and thereby preventing the writing instrument from being lost.