A rolling bearing unit provided with a radially outer ring having an assembly groove, two radially inner rings, two series of rolling elements interposed between the radially outer ring and the radially inner rings and defining between them a first internal grease reservoir. The groove is arranged in an axially intermediate position with respect to the two radially inner rings and a grease dispensing device is housed inside the groove of the radially outer ring and is in turn provided with a second grease reservoir formed inside the radially outer ring and communicating with the first reservoir via the groove. A permeable retaining wall housed is inside the groove so as to retain the grease inside the second reservoir and allow it to flow out towards the first reservoir during operation of the bearing unit, dispensing it in a controlled manner.