In this work, we give sharp asymptotic equivalents in the limit $h\to 0$ of the small eigenvalues of the Witten Laplacian, that is the operator associated with the quadratic form $$ \psi\in H^1_0(\Omega)\mapsto h^2 \int_\Omega \big \vert \nabla \big (e^{\frac 1hf} \psi\big )\big \vert^2\, e^{-\frac 2hf},$$ where $\overline\Omega=\Omega\cup \partial \Omega$ is an oriented $C^\infty$ compact and connected Riemannian manifold with non empty boundary $\partial \Omega$ and $f: \overline \Omega\to \mathbb R$ is a $C^\infty$ Morse function. The function $f$ is allowed to admit critical points on $ \partial \Omega$, which is the main novelty of this work in comparison with the existing literature.