Lighting or signalling device for a motor vehicle comprising: a light guide layer (1) having a top face (1a) and a bottom face (1b), a rear edge (1c) with at least one lobe and an output edge (1d); at least one light-emitting diode (3), mounted on a printed circuit board (B) capable of injecting a beam of light via one face of the guide layer which, by refraction and/or reflection on the faces and on the rear edge, yields an essentially parallel beam, emerging via the output edge (1d); means (F) of fixing the guide layer (1, 1') into a support (D), which means comprise means (S) of clamping the faces (1a, 1b) of the layer between a top element (6) against which the top face (1a) of the layer bears and a bottom component (7) against which the bottom face (1b) bears, while positioning means (P) are provided between the bottom element (7, 7', 7") and the layer (1) and between the various components (6,12) of the device (A).