The invention relates to a device for assembling a casing ring in a watch case middle including bearing surfaces on the inner wall thereof, the casing ring including two housings opening in the lateral face of the casing ring, and in the upper or lower face of the casing ring.The device includes a locking part, capable of pivoting about a vertical axis of rotation passing through said 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 a bearing surface of the inner wall of the case middle so as to hold said casing ring in said case middle.According to the invention, one locking part is snap-fitted in each of the housings and pivots about the vertical axis of rotation.The invention also concerns a watch fitted with such an assembly device.