The electric linear motion actuator includes an electric motor, a drive shaft configured to be rotated by the electric motor, a rotation-to-linear motion converting mechanism configured to convert the rotational motion of the drive shaft to the axial linear motion of an axially movably supported outer ring member, and an electric locking mechanism incorporated in a reduction gear mechanism in which the rotation of a rotor shaft of the electric motor is reduced in speed and transmitted to the drive shaft. When the electric locking mechanism is energized, the power transmission path shifts to the locked state in which power cannot be transmitted, and when the electric locking mechanism is de-energized thereafter, the power transmission path is kept in the locked state. The electric locking mechanism is built in a motor case of the electric motor so that the electric motor and the electric locking mechanism are unitized.