Accessory for windows or doors comprising a base body able to be removably connected to a section bar of a window or door, wherein said section bar has a portion having a bottom which connects to two side edges (202, 203), which extend in a respective flap (204, 205) substantially parallel to the bottom, said flaps (204, 205) delimiting an access slit to the channel defined between said bottom and said side edges (202, 203), wherein said accessory comprises a connection system that is mounted on said base body for the removable connection of said base body to said section bar, wherein said connection system comprises: - an anchoring plate which is suitable to be fixed to the base body of said accessory and which can be inserted in said channel through said slit, wherein said anchoring plate has a first shaped longitudinal edge leaning on the free end of one of said flaps (204, 205) or one of said side edges (202, 203) and a second shaped longitudinal edge opposite to said first shaped longitudinal edge; characterized in that said connection system comprises: - at least one threaded member adapted to attach said anchoring plate to said section bar, - at least one fitting body that is adapted to be rotatably mounted on said base body of said accessory rotating around an axis of rotation substantially orthogonal to said bottom, without possibility of any translation with respect to said base body, in particular without any possibility of any sliding along said axis of rotation, said at least one fitting body having a first fitting projection and a second fitting projection substantially opposite to each other with respect to said axis of rotation, wherein said at least one fitting body is rotatable between a first position, in which said first fitting projection thereof and said second fitting projection thereof can be substantially aligned with the longitudinal axis of said channel without interfering with said anchoring plate and with said section bar, and a second position, in which said first fitting projection thereof is able to be fitted against said second shaped longitudinal edge of said anchoring plate and said second fitting projection thereof is able to be fitted against the side edge (202, 203) of said section bar facing said second shaped longitudinal edge of said anchoring plate, wherein said at least one threaded member is engaged with a corresponding threaded hole which is obtained in said anchoring plate in a through manner from said first shaped longitudinal edge thereof to said second shaped longitudinal edge thereof, wherein said at least one threaded member is suitable to engage, with the free end thereof, against the flap (204, 205) and/or the side edge (202, 203) opposed to the flap (204, 205) or to the side edge (202, 203) against which said first shaped longitudinal edge of said anchoring plate leans. - an anchoring plate which is suitable to be fixed to the base body of said accessory and which can be inserted in said channel through said slit, wherein said anchoring plate has a first shaped longitudinal edge leaning on the free end of one of said flaps (204, 205) or one of said side edges (202, 203) and a second shaped longitudinal edge opposite to said first shaped longitudinal edge; - at least one threaded member adapted to attach said anchoring plate to said section bar, - at least one fitting body that is adapted to be rotatably mounted on said base body of said accessory rotating around an axis of rotation substantially orthogonal to said bottom, without possibility of any translation with respect to said base body, in particular without any possibility of any sliding along said axis of rotation, said at least one fitting body having a first fitting projection and a second fitting projection substantially opposite to each other with respect to said axis of rotation, wherein said at least one fitting body is rotatable between a first position, in which said first fitting projection thereof and said second fitting projection thereof can be substantially aligned with the longitudinal axis of said channel without interfering with said anchoring plate and with said section bar, and a second position, in which said first fitting projection thereof is able to be fitted against said second shaped longitudinal edge of said anchoring plate and said second fitting projection thereof is able to be fitted against the side edge (202, 203) of said section bar facing said second shaped longitudinal edge of said anchoring plate,