A device is provided for assembling a casing ring in a watch case middle including a bearing surface on an inner wall thereof, the casing ring including a housing opening in a lateral face of the casing ring, and in an upper or lower face of the casing ring. The device includes a locking part pivotable about a vertical axis of rotation passing through the upper or lower opening from a first position, in which the locking part is housed in the housing, to a second position, in which the locking part protrudes and abuts against the bearing surface of the inner wall of the case middle so as to hold the casing ring in the case middle. The locking part is snap-fitted in the housing and is pivotable about the vertical axis of rotation. A timepiece and a watch case fitted with the assembly device are also provided.
