A mobile electronic device is provided. The mobile electronic device includes a case having a sidewall; a circuit board mounted in the case, and a side button module attached to the case sidewall. The circuit board includes a switch disposed thereon. The side button module includes a positioning member secured to the case sidewall and a button secured to the positioning member. The button includes a base and an actuating protrusion protruding from the base. The base abuts the positioning member. The actuating protrusion extends through the positioning member and is configured to trigger the switch on the circuit board when actuated. The positioning member is sandwiched between the base and the actuating protrusion.