Yes, it is.
More generally, the following result holds.

**Proposition.** If $R$ is henselian at the maximal ideal $\mathfrak{m}$, then $R[[x_1, \ldots, x_n]]$ is henselian at the maximal ideal lying over $\mathfrak{m}$.

A reference is the paper by N. Sankharan *A Theorem on Henselian Rings*, Canad. Math. Bull. **11** (1968), 275-277. See in particular Corollary 2.

**Remark.** The Proposition above is no longer valid if one takes the polynomial ring instead of the power series ring. For instance, if $K$ is a field than $K$ is henselian but $K[x]$ is not.