  We introduce a new kind of Entanglement Witness which is appropriate for
studying genuine multipartite entanglement in large systems. The witness
operator has a form that fits naturally to quantum states represented by tensor
networks. It opens up novel routes for investigating properties of condensed
matter systems, and gives a new perspective to understand many-body quantum
correlations. We illustrate the potential of this new operator with a GHZ
state, and apply the method to a transverse Ising model with the well-known
approach of Matrix Product States and Matrix Product Operators for a hundred
spins.
