The present invention concerns un rotary joint (10, 10′) apt to transfer a fluid between two entities, one of which is in rotary motion with respect to the other, said rotary joint (10, 10′) being characterised in that it comprises a cylindrical internal element (11), apt to be fixed to said entity in rotary motion, and a plurality of annular external elements (14, 15, 17, 18), which can be coupled and which are self-centering, which can be fitted modularly, said external elements (14, 15, 17, 18) comprising at least two head elements (14), at least two housing elements (15) of respective gaskets (16), two bearing elements (17) and at least one fluid inlet element (18), said at least one fluid inlet element (18) being placed in an intermediate position between said two bearing elements (17), said at least two housing elements (15) of respective gaskets (16) being arranged externally with respect to said two bearing elements (17) and said at least two head elements (14) being arranged externally with respect to said at least two housing elements (15) of respective gaskets (16), said gaskets (16) defining an annular shaped sealed area in the space delimited laterally by the same gaskets (16), internally by said internal element (11) and externally by said external elements (14, 15, 17, 18) comprised between said gaskets (16), said sealed area being accessible on one side through said fluid inlet element (18) and on the other side through at least one fluid passage channel (20, 21) passing through the body of said internal element (11) up to one of the two axial ends.Moreover, the invention concerns pressure regulation systems of a tire (B) comprising said rotary joint (10, 10′).