A linear actuator, particularly for sliding doors and for sliding doors and windows in general, the linear actuator comprising: a supporting body, to be fixed by corresponding fixing means between the two jambs (S1, S2) of a door or of a door or window frame, motor drive means being accommodated and fixed within said supporting body, a driving gear adapted to be rotated by said motor drive means about a rotation axis, a driven rack for meshing with said driving gear, said driven rack being adapted to be fixed to a leaf of the door to be moved in translation, said rotation axis of said driving gear extending parallel to the longitudinal axis of said driven rack, and said supporting body being substantially tubular and telescopic in the direction of said rotation axis of said driving gear, due to the extractability of at least one lateral portion thereof with respect to a central part, characterized in that said supporting body is adapted to be fixed at one end to a jamb by means of a metallic bracket and corresponding threaded elements for fixing to the jamb and to a lateral cover of said supporting body, and at an opposite end by means of a slider that is slidably coupled on said driven rack, said driving gear being constituted by an endless screw, a thread of which is adapted to engage between oblique teeth of the rack, said rack having, at long sides (23, 24) thereof, respective longitudinal grooves (25, 26), which are adapted to accommodate engagement and sliding tabs (27, 28) that protrude from the slider, said slider being fixed to the supporting body at the endless screw and having an opening adapted to allow the engagement of the thread of said endless screw with the rack. a supporting body, to be fixed by corresponding fixing means between the two jambs (S1, S2) of a door or of a door or window frame, motor drive means being accommodated and fixed within said supporting body, a driving gear adapted to be rotated by said motor drive means about a rotation axis, a driven rack for meshing with said driving gear, said driven rack being adapted to be fixed to a leaf of the door to be moved in translation, said rotation axis of said driving gear extending parallel to the longitudinal axis of said driven rack, and said supporting body being substantially tubular and telescopic in the direction of said rotation axis of said driving gear, due to the extractability of at least one lateral portion thereof with respect to a central part, characterized in that said supporting body is adapted to be fixed at one end to a jamb by means of a metallic bracket and corresponding threaded elements for fixing to the jamb and to a lateral cover of said supporting body, and at an opposite end by means of a slider that is slidably coupled on said driven rack, said driving gear being constituted by an endless screw, a thread of which is adapted to engage between oblique teeth of the rack, said rack having, at long sides (23, 24) thereof, respective longitudinal grooves (25, 26), which are adapted to accommodate engagement and sliding tabs (27, 28) that protrude from the slider, said slider being fixed to the supporting body at the endless screw and having an opening adapted to allow the engagement of the thread of said endless screw with the rack.