A device designed for demounting a tire from a rim at least one of whose seats is inclined outwards, the device being in the form of an insert (4) of length (L) with three faces and whose cross-section in a transverse plane is triangular overall with a contour formed by two long sides (41, 42) and a short side (43), the angle between the long sides being close to the angle between the rim seats and the rotation axis, the length of the short side being essentially equal to the height of the projection of the outward-inclined rim seat, this insert (4) comprising on its face intended to come in contact with a bead seat at least one relief (421, 422) designed to interact with said bead seat (11) so as to keep the insert engaged with the bead.