Machine for cutting cardboard tubes, including a cutting unit (CU) with a carriage (7) on which is mounted a blade (6) and a support element (4, 5, 8) for supporting a cardboard tube (1) intended to be transversely cut by the blade (6). The support element (4,5,8) allows the tube (1) to slide along a predetermined direction (A) parallel to its axis longitudinal (x) and to rotate about the same axis, and the carriage (7) is adapted to move bidirectionally the blade (6) parallel to the longitudinal axis (x) of the tube (1). The support element (4,5,8) has, where the cutting unit (CU) is arranged, apart (5) movable from and towards the blade (6) to maintain the tube (1) spaced from the latter and, respectively, for approaching the tube to the same blade (6) during a cutting phase.