An adjustment device for bicycle seat includes a connection portion located close to a top of the seat post and at least one through hole is defined through a wall of the connection portion. An adjustment device is located between the seat connector and the seat post so as to adjust the seat post relative to the seat tube. The adjustment device includes an adjustment path which can communicate with the at least one through hole or be sealed from the at least one through hole. A balance unit is connected to the connection portion and includes a cylinder and a piston ring is movably received in the cylinder to define an inner space of the cylinder into a liquid space and an air space. The cylinder includes a supply path which communicates with the liquid space and the at least one through hole.