We define the Maslov index of a loop tangent to the characteristic foliation of a coisotropic submanifold as the mean Conley-Zehnder index of a path in the group of linear symplectic transformations, incorporating the rotation of the tangent space of the leaf—this is the standard Lagrangian counterpart—and the holonomy of the characteristic foliation. We also show that, with this definition, the Maslov class rigidity extends to the class of the so-called stable coisotropic submanifolds including Lagrangian tori and stable hypersurfaces.