A button assembly includes a keycap, a washer, a positioning member, and a fixing member. The keycap includes a resisting member and an operating member. The operating member extends through a through hole of a sidewall of an electronic device and extends external to the sidewall of the electronic device. The washer is provided between the resisting member and the sidewall to seal a gap between the resisting member and the sidewall. The washer pivots on the operating member. The positioning member is fixed to a bottom of the electronic device. The positioning member separates from the sidewall of the electronic device by a preset distance. The fixing member affixes a flexible circuit board of the electronic device to the positioning member. The fixing member is provided to position a switch of the flexible circuit board abutting against the resisting member at a position opposite to the keycap.