In this article, we provide a Stein type characterization for $G$-normal distributions: Let $\mathcal{N}[\varphi]=\max_{μ\inΘ}μ[\varphi],\ \varphi\in C_{b,Lip}(\mathbb{R}),$ be a sublinear expectation. $\mathcal{N}$ is $G$-normal if and only if for any $\varphi\in C_b^2(\mathbb{R})$, we have \[\int_\mathbb{R}[\frac{x}{2}\varphi'(x)-G(\varphi"(x))]μ^\varphi(dx)=0,\] where $μ^\varphi$ is a realization of $\varphi$ associated with $\mathcal{N}$, i.e., $μ^\varphi\in Θ$ and $μ^\varphi[\varphi]=\mathcal{N}[\varphi]$.