A centering device (10), intended to be used in an automatic bar loader for a lathe, comprises a channel (11) with a rectilinear axis (12) intended for the passage of a bar from the loader to the lathe. The centering device comprises mobile jaws (17) on the sides of the channel (11) for defining an adjustable through-aperture inside the channel. The jaws (17) are mounted in a supporting slider (20) so as to be slidable in the supporting slider (20) radially relative to the channel (11) and the supporting slider (20) is slidable along the channel (11) against the action of elastic means (25) in response to a thrust on the jaws directed towards the said outlet end (16). The slider may thus move between a first position closer to the inlet end (15), in which the jaws are in a first position projecting into the channel with their operative surface (18), and a second position closer to the outlet end (16), in which the jaws may slide inside the supporting slider so as to move radially away from the said position projecting into the channel. A loader with such a centering device is also described.