Device for mixing fluid products comprising a container (3) for a secondary fluid mixable with a primary fluid contained in a vessel (1). The container (3), configured to be applied to the outflow mouth (3) of the vessel (1), has a central path (4) for the flow of the primary fluid and a peripheral chamber (5) containing the secondary fluid and having an outlet hole (8) arranged at a lateral pouring spout (9) of the container (3). A cover (11) provided with a deflector (12) is applied above the central flow path (4) to direct the primary fluid towards the hole (8) for the outlet of the secondary fluid.