A pen-and-electronic device assembly comprises: a barrel; a writing member; and an electronic device. The electronic device includes: a housing having a battery-mounting part including top and bottom walls and a peripheral wall cooperating with the top and bottom walls to define an enclosed battery compartment, the peripheral wall including a first portion fixed to the top and bottom walls and defining an access opening, and a second portion connected detachably to the first portion for covering the access opening; and a circuit unit in the housing. The second portion is fastened to the bottom wall so as to prevent undesired access to the battery compartment.