  Let $d_G(v)$ be the degree of the vertex $v$ in a graph $G$. The Sombor index
of $G$ is defined as $SO(G) =\sum_{uv\in E(G)}\sqrt{d^2_G(u)+d^2_G(v)}$, which
is a new degree-based topological index introduced by Gutman. Let
$\mathscr{T}_{n,\Delta}$ and $\mathscr{U}_{n,\Delta}$ be the set of trees and
unicyclic graphs with $n$ vertices and maximum degree $\Delta$, respectively.
In this paper, the tree and the unicyclic graph with minimum Sombor index among
$\mathscr{T}_{n,\Delta}$ and $\mathscr{U}_{n,\Delta}$ are characterized.
