  Let $(M,g)$ be a smooth connected Riemannian manifold. We show an improvement
of flatness theorem for hypersurfaces of $M$ of bounded nonlocal mean curvature
in the viscosity sense. It implies local $ C^{1,\alpha}$ regularity of these
hypersurfaces provided that they are sufficiently flat. It extends a result of
Caffarelli, Roquejoffre and Savin in the Euclidean setting to the case of
arbitrary Riemannian manifolds.
