Expandable pin for supporting reels of paper material, comprising an outer side (PX) and an inner side (PN), with the inner side (PN) that is adapted to be inserted into a reel (1) of paper material and with the outer side (PX) that remains on the outside of the same reel (1) when the inner side (PN) is inside the reel (1). An outer surface of said inner side (PN) is formed by more sectors (4), each of which is formed by a cylindrical surface portion with a free front part (40). The front parts (40) of said sectors (4) define a substantially circular shape with a diameter varying between a maximum value (4a) and a minimum value (4c). Each of said sectors (4) has a part pivoted on a respective hinge (42; 106) and is connected to moving means apt to determine its rotation around said hinge (42; 106) such that the sectors (4) define a cylindrical shape or a conical shape depending on their rotation around said hinge (42; 106).