An adjustable seat post assembly comprising: a seat post ; a pneumatic cylinder ; a pressure vessel ; a control valve ; and a valve actuator ; a locking device ; wherein the seat post and the pneumatic cylinder extend along a first axis (A1), the seat post being operatively connected to a cylinder tube of the pneumatic cylinder , wherein the pneumatic cylinder comprises a first chamber , which is located on one side of a piston , and an open second chamber , which is located on an opposite side of the piston , wherein the control valve can be actuated via the valve actuator , wherein compressed air can be supplied from the pressure vessel via lines (70;71) and the control valve to the first chamber , wherein a first line leads from the control valve to the first chamber and wherein a compression spring is provided in the open second chamber , which extends in the direction of the first axis (A1) and which acts on the piston , characterized in that the locking device comprises a locking bolt and an adjusting element which are designed in such a way that a displacement of the adjusting element along the first axis (A1) leads to a displacement of the locking bolt in a second axis (A2) perpendicular to the first axis (A1), wherein the locking bolt is arranged in a fixing position in a positioning bore of the seat post and wherein the locking bolt is arranged in a free position outside the positioning bores , and in that the adjusting element is operatively connected to an actuating element of the control valve , so that when the actuating element is actuated, the adjusting element can be actuated at the same time. a seat post ; a pneumatic cylinder ; a pressure vessel ; a control valve ; and a valve actuator ; a locking device ; wherein the seat post and the pneumatic cylinder extend along a first axis (A1), the seat post being operatively connected to a cylinder tube of the pneumatic cylinder , wherein the pneumatic cylinder comprises a first chamber , which is located on one side of a piston , and an open second chamber , which is located on an opposite side of the piston , wherein the control valve can be actuated via the valve actuator , wherein compressed air can be supplied from the pressure vessel via lines (70;71) and the control valve to the first chamber , wherein a first line leads from the control valve to the first chamber and wherein a compression spring is provided in the open second chamber , which extends in the direction of the first axis (A1) and which acts on the piston , characterized in that the locking device comprises a locking bolt and an adjusting element which are designed in such a way that a displacement of the adjusting element along the first axis (A1) leads to a displacement of the locking bolt in a second axis (A2) perpendicular to the first axis (A1), wherein the locking bolt is arranged in a fixing position in a positioning bore of the seat post and wherein the locking bolt is arranged in a free position outside the positioning bores , and in that the adjusting element is operatively connected to an actuating element of the control valve , so that when the actuating element is actuated, the adjusting element can be actuated at the same time. <EOS>