Piloted electromagnetic brake for controlling the tension of the weft threads in weaving machines, in particular of a weft thread which has a high number of knots, of the type including a pair of opposite elastic thin plates between which the weft thread runs, an operated thin plate being adjusted in position by an electromagnetically controlled operating piston, and a resisting thin plate being resistant against an elastic contrast unit, in order to control the intensity of the contrast force between the pair of thin plates. The resisting thin plate and the operated thin plate have fulcrum points in correspondence of a central portion thereof, so as to be able to freely oscillate in a plane containing the weft thread, during the passage of a knot between them, and the fulcrum points are offset by a set length along the weft thread running direction.