A height adjustable seatpost assembly is provided with first and second tubes, a ratchet mechanism and a drive source. The first and second tubes are telescopically arranged. The ratchet mechanism includes a ratchet tooth structure and a pawl structure. The pawl structure is engaged with the ratchet tooth structure in a lock position. The pawl structure is movably coupled with respect to the ratchet tooth structure in a movable position such that the first and second tubes are movably arranged relative to each other in at least one axial direction of the first and second tubes. The drive source moves the pawl structure between the lock position and the movable position in response to operation of the drive source. The drive source further axially moves the first and second tubes relative to each other in response to operation of the drive source.