A motorized bicycle seatpost assembly is basically provided with a first tube, a second tube, a motor and a movement mechanism. The first tube has a first end and a second end. The second tube has a first end and a second end. The first end of the second tube is open such that the second end of the first tube is telescopically disposed in the first end of the second tube. The motor is arranged in at least one of the first tube and the second tube. The movement mechanism is connected to the motor to change a relative axial position between the first and second tubes.