The light-guiding optical unit (1) for a light device of motor vehicles comprises a light guide (2) that comprises at least one routing surface (3), and at least one light source (4) to generate light rays (10). The light-guide (2) further comprises a top surface (12) at least a part of which is constituted by the output surface (15), and a bottom surface (13) opposite the top surface (12) and fitted with a prismatic structure (16) comprising reflective surfaces (7). The routing surface (3) and the reflective surfaces (7) are mutually arranged in such a way that the routing surface (3), by means of routing by refraction on this surface (3) or reflection from this surface (3), directs light rays (10) to the prismatic structure (16) in such a way that it directly lights up only the reflective surfaces (7) with the light rays (10), the reflective surfaces (7) being configured to direct light rays (10) that have fallen onto them this way directly from the routing surface (3) to the output surface (15) in such a way that the light rays (10) can exit from the light guide (2) through the output surface (15).