An electric linear motion actuator includes an outer ring member provided around a central shaft, and planetary rollers disposed between the central shaft and the outer ring member. One of the central shaft and the outer ring member serves as an input member which is rotatable and immovable in the axial direction and rotated by an electric motor, while the other serves as an output member which is rotationally stationary and movable in the axial direction. The input member has a peripheral surface formed with a plurality of annular ribs which are equal in pitch. Each planetary roller has a radially outer surface formed with annular grooves which are equal in pitch to the annular ribs and in which the annular ribs are engaged. The output member has a peripheral surface formed with a helical rib which is equal in pitch to the annular grooves of the planetary rollers.