An adjustable tuning tube for a clarinet is composed of an outer tube and an inner tube. The outer tube has a first upper chamber, a lower chamber, and a middle portion between the first upper chamber and the lower chamber. A button is pivotally mounted in an arcuate slot defined at an outer periphery of the middle portion and has a finger formed at a first end thereof and extending in the outer tube, and multiple first teeth formed on the finger. The inner tube is received in the outer tube, and has multiple second teeth formed at an outer periphery thereof and matching the first teeth of the button.