A lighting device for a vehicle, in particular a headlight for a motor-vehicle, comprising a cup-shaped housing that houses at least one light source, a transparent element arranged to close a front opening of the cup-shaped housing and an optical system also housed within the cup-shaped housing and configured to pick up at least one first light beam generated by said light source and project, in a pre-set direction, a second light beam having a pre-set photometric profile, wherein the light source consists of at least one LED or one LED battery, wherein the optical system comprises a light guide constituted by a flat band delimited between a front face and a rear face thereof, which are substantially flat, and having a first and a second end, opposite to one another, the first end being configured to pick up said at least one first light beam generated by said at least one LED or LED battery, and where the first end of the light guide is provided, on the side opposite the second end, with a transverse edge shaped so as to define at least one first collimating element, for example a concave or convex lens, arranged facing said at least one LED or LED battery and configured to collect within the light guide said at least one first light beam and to make a first collimation thereof towards the second end of the light guide; and wherein said front face of the flat band constituting the light guide comprises an emission section of said second light beam formed on the front face itself, the second end of the light guide being provided, on the side of the rear face of the flat band, with light extracting elements configured to deflect the second light beam through the emission section of the front face so that the second light beam has an emission direction substantially perpendicular to the front face; characterised in that, in combination, i)- the first end of the light guide is provided towards the second end and on the side opposite said transverse edge, with at least one second collimating element facing to, and operatively associated with, said at least one first collimating element and configured to collect the first light beam after the first collimation and to make a second collimation of the first light beam, generating downstream of the at least one second collimating element and towards the second end, said second light beam with a pre-set photometric profile already inside the light guide and uniformly over the entire height of the flat band constituting the same and measured between an upper longitudinal edge and a lower longitudinal edge of the flat band, perpendicularly to these; ii)- the second collimating element is oriented parallel to said front and rear faces (11,12) and perpendicularly to the upper and lower longitudinal edges (16,18); iii)- said light guide has, between said first and second ends (13,14), a first intermediate section arranged downstream of the second collimating element with respect to a trajectory of said at least one first light beam and delimited between two opposite convergent portions of said front and rear faces (11,12) so that the first intermediate section has a thickness, measured perpendicularly to said first and second faces (11,12), progressively reducing towards the second end, but a constant height measured perpendicularly to said upper and lower longitudinal edges (16,18), the light guide extending, downstream of the first intermediate section and towards the second end, with a cross-section having a constant thickness measured perpendicularly to said front and rear faces. i)- the first end of the light guide is provided towards the second end and on the side opposite said transverse edge, with at least one second collimating element facing to, and operatively associated with, said at least one first collimating element and configured to collect the first light beam after the first collimation and to make a second collimation of the first light beam, generating downstream of the at least one second collimating element and towards the second end, said second light beam with a pre-set photometric profile already inside the light guide and uniformly over the entire height of the flat band constituting the same and measured between an upper longitudinal edge and a lower longitudinal edge of the flat band, perpendicularly to these; ii)- the second collimating element is oriented parallel to said front and rear faces (11,12) and perpendicularly to the upper and lower longitudinal edges (16,18); iii)- said light guide has, between said first and second ends (13,14), a first intermediate section arranged downstream of the second collimating element with respect to a trajectory of said at least one first light beam and delimited between two opposite convergent portions of said front and rear faces (11,12) so that the first intermediate section has a thickness, measured perpendicularly to said first and second faces (11,12), progressively reducing towards the second end, but a constant height measured perpendicularly to said upper and lower longitudinal edges (16,18), the light guide extending, downstream of the first intermediate section and towards the second end, with a cross-section having a constant thickness measured perpendicularly to said front and rear faces.