A slot-milling tool includes a slot-milling insert to be on-edge mounted to a toolholder of the slot-milling tool. The insert includes a first side surface, a second side surface, and a plurality of edge surfaces between the first and second side surfaces, each edge surface intersecting with another edge surface at a corner and forming a cutting edge. The first and second side surfaces each have a side insert supporting surface for abutting against an abutment surface in a toolholder, and the insert has reflective symmetry about a central plane extending through the edge surfaces midway between the first and second side surfaces. A chipbreaker is provided on each edge surface and corresponds to each cutting edge, and each chipbreaker includes a recess beneath the cutting edge and a portion projecting toward the cutting edge, the projecting portion defining first and second chip deflecting recesses between the projecting portion and the first and second side surfaces, respectively.