A height adjustment device for a bicycle saddle is provided and essentially includes a hydraulic cylinder, an adjustment disk, and a driving mechanism. An annular guiding groove is disposed on the adjustment disk and has a bottom whose height decreases from one end of the annular guiding groove to the other end of the annular guiding groove. When the adjustment disk is being driven by a motor, the annular guiding groove enables a relief valve of the hydraulic cylinder to turn on and turn off according to the level of the accumulated pressure, so as to fine-tune saddle height.