A vertex-degree-based molecular structure descriptor was introduced by Ivan Gutman, named the Sombor index. The Sombor index of a graph $G$ is defined as $SO(G)=\sum _{uv\in E(G)}\sqrt{d_G(u{)}^2+d_G(v{)}^2}$, where $d_G(u)$ denotes the degree of the vertex $u$ in $G$. In this paper we determine the extremal values of Sombor index of tricyclic graphs.