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 $\mathfrak{m}$.

See N. Sankharan, [*A theorem on Henselian Rings*][1], Canad. Math. Bull. **11** 275-277 (1968), in particular Corollary 2.
 


  [1]: http://cms.math.ca/10.4153/CMB-1968-032-4