An electric power steering system includes: an assist motor that generate assist force for assisting steering of a steering member; a motor drive circuit that controls driving of the assist motor; a capacitor that is electrically connected to a main power supply that supplies electric power to the assist motor, the motor drive circuit, and electrical components; and a controller that controls charging and discharging of the capacitor. The controller discharges electricity to the electrical components when an amount of electric charge remaining in the auxiliary power supply is equal to or greater than a first threshold value in an engine start-up period in a vehicle.