The author defines 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 symplectic transformations, incorporating the rotation of the tangent space of the leaf and the holonomy of the characteristic foliation. Its properties include homotopy invariance, recapping and homogeneity. The author also shows that the Maslov class rigidity extends to the class of the so-called stable coisotropic submanifolds including Lagrangian tori and stable hypersurfaces.