Ball bearing unit, particularly a deep groove ball bearing unit, comprising an outer ring with a radially inner surface providing a radially outer raceway, an inner ring with a radially outer surface providing a radially inner raceway, a set of bearing balls radially interposed between the outer and the inner raceways, and a cage, the cage comprising: - a plurality of pockets each delimited by a respective spherical surface having a centre lying in a radial mid-plane of the bearing, - two half-cages (51, 52) substantially symmetrical about the mid-plane, and connected to one another, each half-cage having an annular body and a plurality of partially hemispherical cavities (64, 66, 68) axially facing the hemispherical cavities of the opposite half-cage so as to define therewith the pockets; - connecting means (69, 70) associated with each cage for connecting together the two half-cages; - for each half-cage (51, 52), two substantially concentric and circumferentially continuous radial ridges (56, 57) extending radially in opposite directions from each annular body, of which the first ridge extends in a radially outward direction for sealingly engaging a groove formed in the outer ring of the bearing unit, and the second ridge extends in a radially inward direction for sealingly engaging a groove formed in the inner ring of the bearing unit; characterized in that the outer ring provides, on both sides of the raceway, a respective intermediate annular groove between the raceway and the groove engaged by the half-cage; the first and second ridges (56, 57) extend from a respective edge (58, 59) projecting in an axially inward direction from a radially outer peripheral portion and a radially inner peripheral portion, respectively, of the annular body; each half-cage (51, 52) provides two concentric annular receptacles (60, 61) open towards the mid-plane, each bounded by a respective edge (58, 59), and adapted to serve as reservoirs for lubricating grease; the annular receptacles (60, 61) each have a conical surface (62, 63), formed by the respective edges (58, 59) and diverging towards the mid-plane; and that the respective intermediate groove is axially adjacent to the conical surface of the respective edge, so as to receive lubricating grease from the respective circumferential receptacle. - a plurality of pockets each delimited by a respective spherical surface having a centre lying in a radial mid-plane of the bearing, - two half-cages (51, 52) substantially symmetrical about the mid-plane, and connected to one another, each half-cage having an annular body and a plurality of partially hemispherical cavities (64, 66, 68) axially facing the hemispherical cavities of the opposite half-cage so as to define therewith the pockets; - connecting means (69, 70) associated with each cage for connecting together the two half-cages; - for each half-cage (51, 52), two substantially concentric and circumferentially continuous radial ridges (56, 57) extending radially in opposite directions from each annular body, of which the first ridge extends in a radially outward direction for sealingly engaging a groove formed in the outer ring of the bearing unit, and the second ridge extends in a radially inward direction for sealingly engaging a groove formed in the inner ring of the bearing unit;