A locking device in an office chair structure of the type with articulation permitting the movement of the adjustable seat and seat back, with a mobile base from which a column rises, which is adjustable in height, and which supports a seat plane and a hinged seat back. The structure of the office chair includes a seat, attached at the front to the corresponding front side of a central support body secured to the top of the adjustable column along the vertical axis; a central support body, which, at the back, keeps in place the lower ends of the two support uprights of the armrests and a seat back connected to the seat, swinging and linked, near to the back part, to the central support body by means of a compression spring; Coaxially to the compression spring, there is a rod, one side of which is integral with the back rest, while the other side is axially mobile with respect to a static connecting body linked to the central support body of the chair.