The invention concerns a lighting and/or signalling device for a motor vehicle comprising at least one light source emitting a light beam and at least one optical guide in which the light beam propagates, said optical guide comprising an output face for the light beam, and another reflection face, opposite to the output face, having a serrated profile forming a reflection face for the light beam, notably comprising a series of prisms, each prism forming, with the following prism (8), a bottom angle (D). At least one bottom angle of the reflection face is truncated, and/or in that the output face has a profile comprising flutes.