Abstract: A bicycle seatpost assembly comprises a first tube, a second tube, and a positioning structure. The second tube has a distal end to be attached to a bicycle seat and a proximal end opposite to the distal end. The second tube is configured to be telescopically received in the first tube. The positioning structure is configured to position the first tube and the second tube relative to each other. The positioning structure comprises a fluid chamber. The fluid chamber comprises a first chamber, a second chamber, and a movable piston. The first chamber is filled with a first liquid. The second chamber includes a second liquid and gas. The second chamber is disposed closer to the distal end of the second tube than the first chamber. The movable piston divides the fluid chamber into the first chamber and the second chamber. The movable piston is movable within the fluid chamber.