A locking device for an adjustable steering column assembly of a motor vehicle is described, comprising an operating unit which has at least one closed and one open position, a control element coupled to the operating unit, and at least one eccentric part which is actuated by the control element. A retaining part is provided, which interacts with the eccentric part in the closed position, wherein a locking portion of the eccentric part engages on an engaging portion of the retaining part in the closed position. The eccentric part has a force transmission portion which is substantially opposite to the locking portion, wherein the force transmission portion in the closed position rests against a support surface lying substantially opposite to the engaging portion. Furthermore, a steering column assembly is described.