An adjustable headband device for a helmet has a base, a headband, a driven wheel, a resilient element, a cover plate and an adjusting knob. The headband is mounted on the base and has two toothed racks. The driven wheel has a gear engaging the toothed racks, multiple engaging tabs and multiple ratchet pawls. The cover plate is mounted on the base and has multiple ratchet teeth selectively engaging the ratchet pawls. The adjusting knob of the adjustable headband device rotates to control increase and decrease of an inner diameter defined by the looped headband without being depressed.
