An adjustable bicycle seat assembly includes an adjusting unit which includes a top end member connected to the seat post and an adjusting tube is connected to a lower end of the top end member. An inner space is defined in the adjusting tube and an outer space is defined between the adjusting tube and the seat post. A path is defined between the adjusting tube and the top end member so as to communicate with the inner space and the outer space. An axle is movably received in the path and removably seals the path. An anti-rotation device is located between the seat post and the seat tube so as to prevent the seat post from rotating relative to the seat tube.