The present invention provides a core piece and a reactor that have excellent bondability to a resin portion and are capable of reducing the generation of eddy currents. The core piece is a core piece that constitutes a magnetic core disposed within or outside a coil formed of a wound wire, the core piece including an end surface that is orthogonal to a magnetic flux flowing through the coil and to which a resin portion is bonded, wherein the end surface has an intersecting groove in which a plurality of grooves intersect without forming a loop. The core piece may be, for example, a powder compact made of metal particles and an insulating material present between the metal particles. The core piece may constitute, for example, a portion of the magnetic core, the portion being disposed within the coil.