A device for cutting the weft yarn in weaving looms comprises a fixed-blade and movable-blade cutting group which has associated motor means (8) for the cutting movement of the movable blade in order to perform cutting of the weft yarn. The fixed and movable blades are each mounted on the end of respective elongated blade-carrying arms (9, 10) which extend respectively above and below the fabric holder (3) of the loom, as far as a cutting zone located beyond the leading edge of the fabric holder (3) itself in the direction of the reed (2), a zone in which said blades overlap so as to perform cutting of the weft.