  We study the task of efficiently sampling from a Gibbs distribution $d \pi^*
= e^{-h} d {vol}_g$ over a Riemannian manifold $M$ via (geometric) Langevin
MCMC; this algorithm involves computing exponential maps in random Gaussian
directions and is efficiently implementable in practice. The key to our
analysis of Langevin MCMC is a bound on the discretization error of the
geometric Euler-Murayama scheme, assuming $\nabla h$ is Lipschitz and $M$ has
bounded sectional curvature. Our error bound matches the error of Euclidean
Euler-Murayama in terms of its stepsize dependence. Combined with a contraction
guarantee for the geometric Langevin Diffusion under Kendall-Cranston coupling,
we prove that the Langevin MCMC iterates lie within $\epsilon$-Wasserstein
distance of $\pi^*$ after $\tilde{O}(\epsilon^{-2})$ steps, which matches the
iteration complexity for Euclidean Langevin MCMC. Our results apply in general
settings where $h$ can be nonconvex and $M$ can have negative Ricci curvature.
Under additional assumptions that the Riemannian curvature tensor has bounded
derivatives, and that $\pi^*$ satisfies a $CD(\cdot,\infty)$ condition, we
analyze the stochastic gradient version of Langevin MCMC, and bound its
iteration complexity by $\tilde{O}(\epsilon^{-2})$ as well.
