The problem of finding a curve whose principal normals are also the principal normals of another curve was apparently first proposed by Saint-Venant but solved by J. Bertrand. Such curves were referred to as ‘Bertrand offsets’. In this paper, a generalization of the theory of Bertrand curves is presented for ruled surfaces in Minkowski space $\mathbb R^3_1$. Using lines instead of points, two ruled surfaces which are offset in the sense of Bertrand are defined. The obtained results are illustrated by computer-aided examples.