An armrest adjustment device includes an armrest arranged at a top surface of an armrest support. The lower part of the armrest support is connected to a chair seat. The armrest can be adjusted to move forward and backward in relation to the chair seat. A first component is mounted in the armrest. The first component is assembled with and fixed on the armrest support so that the armrest can be moved forward and backward in relation to the first component. Moreover, a second component is arranged between the armrest and the armrest support. Instead of the first component, the second component is assembled with and fixed on the armrest support. Thus the armrest is moved in the left/right direction in relation to the second component.