The present invention concerns an anti-loosening fastening system (1) comprising: a screw (2) with a head (5), an underhead surface (6) and a shank (7) having a longitudinal rotation axis (X)； an element to be fastened (3) having a hole (8) intended to be engaged by the shank (7), and a leaning surface (9) surrounding the hole (8), wherein the hole (8) is coaxial with the longitudinal rotation axis (X) of the shank (7) when the shank (7) engages the hole (8)； and an annular element (4) intended to be interposed between the underhead surface (6) and the leaning surface (9)； wherein the underhead surface (6) comprises a plurality of first grooves (10), the leaning surface (9) comprises a plurality of second grooves (11), and wherein the annular element (4) is of a plastic material.
