A fastener device for fastening a dispenser member (4) on a neck (11) of a fluid reservoir (1), the device including: a fastener ring (2) including a reception mechanism (21) and a fastener mechanism (23) for engaging around the neck (11) of the reservoir (1); and a blocking hoop (3) engaged around the ring (2) and movable relative to the ring between a provisional pre-assembled position and a final assembled position. The hoop (3) includes a clearance section (32) that, in the provisional position, is disposed around the fastener mechanism (23), but with an intermediate clearance gap (E), so as to enable the fastener mechanism (23) to move radially outwards without being hindered by the hoop (3), and is offset axially in the final position relative to the fastener mechanism (23), so as to enable the hoop to block the ring (2) on the neck (11).