  In the present paper, we first give a characterization for Bongartz
completion in $\tau$-tilting theory via $c$-vectors. Motivated by this
characterization, we give the definition of Bongartz completion in cluster
algebras using $c$-vectors. Then we prove the existence and uniqueness of
Bongartz completion in cluster algebras. We also prove that Bongartz completion
admits certain commutativity. We give two applications for Bongartz completion
in cluster algebras. As the first application, we prove the full subquiver of
the exchange quiver (or known as oriented exchange graph) of a cluster algebra
$\mathcal A$ whose vertices consist of seeds of $\mathcal A$ containing
particular cluster variables is isomorphic to the exchange quiver of another
cluster algebra. As the second application, we prove that in a cluster Poisson
algebra $\mathcal X_\bullet$, each cluster Poisson seed (up to seed
equivalence) of $\mathcal X_\bullet$ is uniquely determined by its negative
cluster Poisson variables.
