A bicycle seatpost assembly comprises a first cylinder, a second cylinder, a positioning structure, and a mechanical position informing structure. The second cylinder is configured to be telescopically received in the first cylinder. The positioning structure is configured to relatively position the first cylinder and the second cylinder such that a positional relationship between the first cylinder and the second cylinder is continuously adjustable within an adjustable position range of the bicycle seatpost assembly. The mechanical position informing structure is configured to inform a user that at least one of the first and second cylinders reaches a reference position. The reference position is defined between a maximum-length position and a minimum-length position of the adjustable position range.