Yes, indeed. The electrostatic potential $V$ satisfies
$$\nabla^{2}V\left(\vec{r}\right)=-\frac{\rho\left(\vec{r}\right)}{\varepsilon_{0}}$$
so in a region $U\subset\mathbb{R}^{3}$ in which $\rho=0$, one also has
$$\nabla^{2}V\left(\vec{r}\right)=0$$