In this paper, we give a new characterization of the cut locus of a
point on a compact Riemannian manifold as the zero set of the optimal
transport density solution of the Monge-Kantorovich equations, a PDE
formulation of the optimal transport problem with cost equal to the
geodesic distance. Combining this result with an optimal transport
numerical solver, based on the so-called dynamical Monge-Kantorovich
approach, we propose a novel framework for the numerical approximation
of the cut locus of a point in a manifold. We show the applicability
of the proposed method on a few examples settled on 2d-surfaces
embedded in $R^{3}$, and discuss advantages and limitations.