An armrest device includes an armrest mount, an armrest member, and a locking unit. The armrest mount and the armrest member are respectively formed with pivotally interconnected first and second pivot connecting portions such that the armrest member is pivotable relative to the armrest mount between first and second positions. The locking unit is mounted operably in the second pivot connecting portion, and includes a press member accessible externally of the second pivot connecting portion, and a locking tab unit projecting from the press member. The press member is operable to move from a locking position, where the locking tab unit engages the first pivot connecting portion to arrest pivoting movement of the armrest member, to an unlocking position, where the locking tab unit is disengaged from the first pivot connecting portion to permit pivoting movement of the armrest member between the first and second positions.