A conductivity sensor for measuring conductivity of a medium surrounding the conductivity sensor includes a first toroidal coil bounding a medium-receiving passageway and serving for inducing an electrical current in the medium, and a second toroidal coil also bounding the passageway and serving for registering a magnetic field produced by the electrical current. At least one of the toroidal coils has a plurality of first conductor segments, which extend in a plane of a multi-ply circuit board, a plurality of second conductor segments, which extend in a second plane of the circuit board, and a plurality of through-contacts, which connect the first conductor segments with the second conductor segments, wherein the first conductor segments, the second conductor segments and the through-contacts form, together, the windings of a toroidal coil.